diff --git a/docs/web/docs/features/proposals/iterator-join.md b/docs/web/docs/features/proposals/iterator-join.md
new file mode 100644
index 000000000000..88cc8df32d94
--- /dev/null
+++ b/docs/web/docs/features/proposals/iterator-join.md
@@ -0,0 +1,28 @@
+# Iterator join
+[Specification](https://bakkot.github.io/proposal-iterator-join/)\
+[Proposal repo](https://github.com/bakkot/proposal-iterator-join)
+
+## Modules
+[`esnext.iterator.join`](https://github.com/zloirock/core-js/blob/v4/packages/core-js/modules/esnext.iterator.join.js)
+
+## Built-ins signatures
+```ts
+class Iterator {
+  join(separator?: string | undefined): string;
+}
+```
+
+## [Entry points]({docs-version}/docs/usage#h-entry-points)
+```ts
+core-js/proposals/iterator-join
+core-js(-pure)/full/iterator/join
+```
+
+## Examples
+```js
+const digits = () => [1, 2, 3].values();
+digits().join();  // => '1,2,3'
+
+const words = () => ['Hello', 'core-js'].values();
+words().join(' ');  // => 'Hello core-js'
+```
diff --git a/docs/web/docs/menu.json b/docs/web/docs/menu.json
index 6d134788fc38..881070958826 100644
--- a/docs/web/docs/menu.json
+++ b/docs/web/docs/menu.json
@@ -341,6 +341,10 @@
               {
                 "title": "Function.prototype.demethodize",
                 "url": "{docs-version}/docs/features/proposals/function-prototype-demethodize"
+              },
+              {
+                "title": "Iterator join",
+                "url": "{docs-version}/docs/features/proposals/iterator-join"
               }
             ]
           }
diff --git a/packages/core-js-compat/src/data.mjs b/packages/core-js-compat/src/data.mjs
index 0d886b963c81..c066e9fcbf0c 100644
--- a/packages/core-js-compat/src/data.mjs
+++ b/packages/core-js-compat/src/data.mjs
@@ -2347,6 +2347,8 @@ export const data = {
   },
   'esnext.iterator.concat': {
   },
+  'esnext.iterator.join': {
+  },
   'esnext.iterator.range': {
   },
   'esnext.iterator.to-async': {
diff --git a/packages/core-js-pure/package.json b/packages/core-js-pure/package.json
index 2c70dad28c41..947a1b84a72b 100644
--- a/packages/core-js-pure/package.json
+++ b/packages/core-js-pure/package.json
@@ -582,6 +582,8 @@
     "./proposals/iterator-helpers.js": "./proposals/iterator-helpers.js",
     "./proposals/iterator-chunking": "./proposals/iterator-chunking.js",
     "./proposals/iterator-chunking.js": "./proposals/iterator-chunking.js",
+    "./proposals/iterator-join": "./proposals/iterator-join.js",
+    "./proposals/iterator-join.js": "./proposals/iterator-join.js",
     "./proposals/iterator-range": "./proposals/iterator-range.js",
     "./proposals/iterator-range.js": "./proposals/iterator-range.js",
     "./proposals/iterator-sequencing": "./proposals/iterator-sequencing.js",
diff --git a/packages/core-js/modules/esnext.iterator.join.js b/packages/core-js/modules/esnext.iterator.join.js
new file mode 100644
index 000000000000..bfe46ef53819
--- /dev/null
+++ b/packages/core-js/modules/esnext.iterator.join.js
@@ -0,0 +1,33 @@
+'use strict';
+var $ = require('../internals/export');
+var $toString = require('../internals/to-string');
+var anObject = require('../internals/an-object');
+var getIteratorDirect = require('../internals/get-iterator-direct');
+var isNullOrUndefined = require('../internals/is-null-or-undefined');
+var iterate = require('../internals/iterate');
+var iteratorClose = require('../internals/iterator-close');
+var uncurryThis = require('../internals/function-uncurry-this');
+
+var $join = uncurryThis([].join);
+var push = uncurryThis([].push);
+
+// `Iterator.prototype.join` method
+// https://bakkot.github.io/proposal-iterator-join/
+// dependency: es.iterator.constructor
+$({ target: 'Iterator', proto: true, real: true, forced: true }, {
+  join: function join(separator) {
+    var O = anObject(this);
+    var sep;
+    try {
+      sep = separator === undefined ? ',' : $toString(separator);
+    } catch (error) {
+      iteratorClose(O, 'throw', error);
+    }
+    var result = [];
+    var iterated = getIteratorDirect(O);
+    iterate(iterated, function (value) {
+      push(result, isNullOrUndefined(value) ? '' : $toString(value));
+    }, { IS_RECORD: true });
+    return $join(result, sep);
+  },
+});
diff --git a/packages/core-js/package.json b/packages/core-js/package.json
index c1cf3a85ff30..383e1655697a 100644
--- a/packages/core-js/package.json
+++ b/packages/core-js/package.json
@@ -576,6 +576,8 @@
     "./proposals/iterator-helpers.js": "./proposals/iterator-helpers.js",
     "./proposals/iterator-chunking": "./proposals/iterator-chunking.js",
     "./proposals/iterator-chunking.js": "./proposals/iterator-chunking.js",
+    "./proposals/iterator-join": "./proposals/iterator-join.js",
+    "./proposals/iterator-join.js": "./proposals/iterator-join.js",
     "./proposals/iterator-range": "./proposals/iterator-range.js",
     "./proposals/iterator-range.js": "./proposals/iterator-range.js",
     "./proposals/iterator-sequencing": "./proposals/iterator-sequencing.js",
diff --git a/scripts/build-entries/entries-definitions.mjs b/scripts/build-entries/entries-definitions.mjs
index 6d42ca6584c1..924ce71880ce 100644
--- a/scripts/build-entries/entries-definitions.mjs
+++ b/scripts/build-entries/entries-definitions.mjs
@@ -1306,6 +1306,18 @@ export const features = {
     namespace: 'Iterator',
     name: 'forEach',
   },
+  'iterator/join': {
+    modules: ['esnext.iterator.join'],
+    template: $uncurried,
+    namespace: 'Iterator',
+    name: 'join',
+  },
+  'iterator/prototype/join': {
+    modules: ['esnext.iterator.join'],
+    template: $prototype,
+    namespace: 'Iterator',
+    name: 'join',
+  },
   'iterator/map': {
     modules: ['es.iterator.map'],
     template: $uncurried,
@@ -3457,6 +3469,13 @@ export const proposals = {
       'esnext.iterator.windows',
     ],
   },
+  'iterator-join': {
+    link: 'https://github.com/bakkot/proposal-iterator-join',
+    stage: 0,
+    modules: [
+      'esnext.iterator.join',
+    ],
+  },
   'iterator-range': {
     link: 'https://github.com/tc39/proposal-Number.range',
     stage: 2,
diff --git a/tests/babel-plugin/fixtures/entry-global/import-full-chrome-135-v4.0/output.mjs b/tests/babel-plugin/fixtures/entry-global/import-full-chrome-135-v4.0/output.mjs
index 60b28c8d0b38..4dc0297d985c 100644
--- a/tests/babel-plugin/fixtures/entry-global/import-full-chrome-135-v4.0/output.mjs
+++ b/tests/babel-plugin/fixtures/entry-global/import-full-chrome-135-v4.0/output.mjs
@@ -33,6 +33,7 @@ import "core-js/modules/esnext.function.demethodize";
 import "core-js/modules/esnext.function.metadata";
 import "core-js/modules/esnext.iterator.chunks";
 import "core-js/modules/esnext.iterator.concat";
+import "core-js/modules/esnext.iterator.join";
 import "core-js/modules/esnext.iterator.range";
 import "core-js/modules/esnext.iterator.to-async";
 import "core-js/modules/esnext.iterator.windows";
diff --git a/tests/babel-plugin/fixtures/entry-global/import-full-chrome-135-v4.999/output.mjs b/tests/babel-plugin/fixtures/entry-global/import-full-chrome-135-v4.999/output.mjs
index 60b28c8d0b38..4dc0297d985c 100644
--- a/tests/babel-plugin/fixtures/entry-global/import-full-chrome-135-v4.999/output.mjs
+++ b/tests/babel-plugin/fixtures/entry-global/import-full-chrome-135-v4.999/output.mjs
@@ -33,6 +33,7 @@ import "core-js/modules/esnext.function.demethodize";
 import "core-js/modules/esnext.function.metadata";
 import "core-js/modules/esnext.iterator.chunks";
 import "core-js/modules/esnext.iterator.concat";
+import "core-js/modules/esnext.iterator.join";
 import "core-js/modules/esnext.iterator.range";
 import "core-js/modules/esnext.iterator.to-async";
 import "core-js/modules/esnext.iterator.windows";
diff --git a/tests/babel-plugin/fixtures/entry-global/import-full-ie11-v4.0/output.mjs b/tests/babel-plugin/fixtures/entry-global/import-full-ie11-v4.0/output.mjs
index 9d189725779d..802c02ed5444 100644
--- a/tests/babel-plugin/fixtures/entry-global/import-full-ie11-v4.0/output.mjs
+++ b/tests/babel-plugin/fixtures/entry-global/import-full-ie11-v4.0/output.mjs
@@ -299,6 +299,7 @@ import "core-js/modules/esnext.function.demethodize";
 import "core-js/modules/esnext.function.metadata";
 import "core-js/modules/esnext.iterator.chunks";
 import "core-js/modules/esnext.iterator.concat";
+import "core-js/modules/esnext.iterator.join";
 import "core-js/modules/esnext.iterator.range";
 import "core-js/modules/esnext.iterator.to-async";
 import "core-js/modules/esnext.iterator.windows";
diff --git a/tests/babel-plugin/fixtures/entry-global/import-full-ie11-v4.999/output.mjs b/tests/babel-plugin/fixtures/entry-global/import-full-ie11-v4.999/output.mjs
index 9d189725779d..802c02ed5444 100644
--- a/tests/babel-plugin/fixtures/entry-global/import-full-ie11-v4.999/output.mjs
+++ b/tests/babel-plugin/fixtures/entry-global/import-full-ie11-v4.999/output.mjs
@@ -299,6 +299,7 @@ import "core-js/modules/esnext.function.demethodize";
 import "core-js/modules/esnext.function.metadata";
 import "core-js/modules/esnext.iterator.chunks";
 import "core-js/modules/esnext.iterator.concat";
+import "core-js/modules/esnext.iterator.join";
 import "core-js/modules/esnext.iterator.range";
 import "core-js/modules/esnext.iterator.to-async";
 import "core-js/modules/esnext.iterator.windows";
diff --git a/tests/babel-plugin/fixtures/entry-global/require-full-chrome-135-v4.0/output.mjs b/tests/babel-plugin/fixtures/entry-global/require-full-chrome-135-v4.0/output.mjs
index 60b28c8d0b38..4dc0297d985c 100644
--- a/tests/babel-plugin/fixtures/entry-global/require-full-chrome-135-v4.0/output.mjs
+++ b/tests/babel-plugin/fixtures/entry-global/require-full-chrome-135-v4.0/output.mjs
@@ -33,6 +33,7 @@ import "core-js/modules/esnext.function.demethodize";
 import "core-js/modules/esnext.function.metadata";
 import "core-js/modules/esnext.iterator.chunks";
 import "core-js/modules/esnext.iterator.concat";
+import "core-js/modules/esnext.iterator.join";
 import "core-js/modules/esnext.iterator.range";
 import "core-js/modules/esnext.iterator.to-async";
 import "core-js/modules/esnext.iterator.windows";
diff --git a/tests/babel-plugin/fixtures/entry-global/require-full-chrome-135-v4.999/output.mjs b/tests/babel-plugin/fixtures/entry-global/require-full-chrome-135-v4.999/output.mjs
index 60b28c8d0b38..4dc0297d985c 100644
--- a/tests/babel-plugin/fixtures/entry-global/require-full-chrome-135-v4.999/output.mjs
+++ b/tests/babel-plugin/fixtures/entry-global/require-full-chrome-135-v4.999/output.mjs
@@ -33,6 +33,7 @@ import "core-js/modules/esnext.function.demethodize";
 import "core-js/modules/esnext.function.metadata";
 import "core-js/modules/esnext.iterator.chunks";
 import "core-js/modules/esnext.iterator.concat";
+import "core-js/modules/esnext.iterator.join";
 import "core-js/modules/esnext.iterator.range";
 import "core-js/modules/esnext.iterator.to-async";
 import "core-js/modules/esnext.iterator.windows";
diff --git a/tests/babel-plugin/fixtures/entry-global/require-full-ie11-v4.0/output.mjs b/tests/babel-plugin/fixtures/entry-global/require-full-ie11-v4.0/output.mjs
index 9d189725779d..802c02ed5444 100644
--- a/tests/babel-plugin/fixtures/entry-global/require-full-ie11-v4.0/output.mjs
+++ b/tests/babel-plugin/fixtures/entry-global/require-full-ie11-v4.0/output.mjs
@@ -299,6 +299,7 @@ import "core-js/modules/esnext.function.demethodize";
 import "core-js/modules/esnext.function.metadata";
 import "core-js/modules/esnext.iterator.chunks";
 import "core-js/modules/esnext.iterator.concat";
+import "core-js/modules/esnext.iterator.join";
 import "core-js/modules/esnext.iterator.range";
 import "core-js/modules/esnext.iterator.to-async";
 import "core-js/modules/esnext.iterator.windows";
diff --git a/tests/babel-plugin/fixtures/entry-global/require-full-ie11-v4.999/output.mjs b/tests/babel-plugin/fixtures/entry-global/require-full-ie11-v4.999/output.mjs
index 9d189725779d..802c02ed5444 100644
--- a/tests/babel-plugin/fixtures/entry-global/require-full-ie11-v4.999/output.mjs
+++ b/tests/babel-plugin/fixtures/entry-global/require-full-ie11-v4.999/output.mjs
@@ -299,6 +299,7 @@ import "core-js/modules/esnext.function.demethodize";
 import "core-js/modules/esnext.function.metadata";
 import "core-js/modules/esnext.iterator.chunks";
 import "core-js/modules/esnext.iterator.concat";
+import "core-js/modules/esnext.iterator.join";
 import "core-js/modules/esnext.iterator.range";
 import "core-js/modules/esnext.iterator.to-async";
 import "core-js/modules/esnext.iterator.windows";
diff --git a/tests/compat/tests.js b/tests/compat/tests.js
index 6490d28da986..25f95e877205 100644
--- a/tests/compat/tests.js
+++ b/tests/compat/tests.js
@@ -1837,6 +1837,9 @@ GLOBAL.tests = {
   'esnext.iterator.concat': function () {
     return Iterator.concat;
   },
+  'esnext.iterator.join': function () {
+    return Iterator.prototype.join;
+  },
   'esnext.iterator.range': function () {
     return Iterator.range;
   },
diff --git a/tests/entries/unit.mjs b/tests/entries/unit.mjs
index c695ca4cdd1f..400fc484ac48 100644
--- a/tests/entries/unit.mjs
+++ b/tests/entries/unit.mjs
@@ -742,6 +742,7 @@ for (PATH of ['@core-js/pure', 'core-js']) {
     ok(typeof load(NS, 'iterator/zip') == 'function');
     ok(typeof load(NS, 'iterator/zip-keyed') == 'function');
     ok(typeof load(NS, 'iterator/chunks') == 'function');
+    ok(typeof load(NS, 'iterator/join') == 'function');
     ok(typeof load(NS, 'iterator/to-async') == 'function');
     ok(typeof load(NS, 'iterator/windows') == 'function');
     ok(typeof load(NS, 'iterator/prototype/chunks') == 'function');
@@ -820,6 +821,7 @@ for (PATH of ['@core-js/pure', 'core-js']) {
   load('proposals/iterator-range');
   load('proposals/iterator-sequencing');
   load('proposals/iterator-chunking');
+  load('proposals/iterator-join');
   load('proposals/joint-iteration');
   load('proposals/json-parse-with-source');
   load('proposals/math-clamp');
diff --git a/tests/eslint/eslint.config.js b/tests/eslint/eslint.config.js
index d366a323e4cb..2418696cbd77 100644
--- a/tests/eslint/eslint.config.js
+++ b/tests/eslint/eslint.config.js
@@ -1272,6 +1272,7 @@ const forbidCompletelyNonExistentBuiltIns = {
   ] }],
   'es/no-nonstandard-iterator-prototype-properties': [ERROR, { allow: [
     'chunks',
+    'join',
     'sliding',
     'toAsync',
     'windows',
diff --git a/tests/unit-global/esnext.iterator.join.js b/tests/unit-global/esnext.iterator.join.js
new file mode 100644
index 000000000000..21957b54c977
--- /dev/null
+++ b/tests/unit-global/esnext.iterator.join.js
@@ -0,0 +1,40 @@
+import { createIterator } from '../helpers/helpers.js';
+
+QUnit.test('Iterator#join', assert => {
+  const { join } = Iterator.prototype;
+  const symbol = Symbol('a');
+
+  assert.isFunction(join);
+  assert.arity(join, 1);
+  assert.name(join, 'join');
+  assert.looksNative(join);
+  assert.nonEnumerable(Iterator.prototype, 'join');
+
+  const observableReturn = {
+    return() {
+      this.called = true;
+      return { done: true, value: undefined };
+    },
+  };
+  const itObservable1 = createIterator([1, 2, 3], observableReturn);
+
+  assert.strictEqual(join.call(itObservable1), '1,2,3', 'without separator attribute');
+  assert.true(itObservable1.called, 'iterator closes');
+  assert.strictEqual(join.call(createIterator([1, 2, 3]), '-'), '1-2-3', 'with separator attribute');
+  assert.strictEqual(join.call(createIterator([1, null, 3]), '-'), '1--3', 'skips nullish values');
+  assert.strictEqual(join.call(createIterator([1, undefined, 3]), '-'), '1--3', 'skips undefined values');
+  assert.strictEqual(join.call(createIterator([])), '', 'empty iterator');
+
+  assert.throws(() => join.call(''), TypeError, 'iterable non-object this');
+  assert.throws(() => join.call(undefined), TypeError, 'non-iterable-object this #1');
+  assert.throws(() => join.call(null), TypeError, 'non-iterable-object this #2');
+  assert.throws(() => join.call(5), TypeError, 'non-iterable-object this #3');
+
+  const itObservable2 = createIterator([1, 2, 3], observableReturn);
+  assert.throws(() => join.call(itObservable2, symbol), TypeError, 'throws on symbol separator');
+  assert.true(itObservable2.called, 'iterator closes on argument validation error');
+
+  const itObservable3 = createIterator([1, symbol, 3], observableReturn);
+  assert.throws(() => join.call(itObservable3), TypeError, 'throws on non-stringable value');
+  assert.true(itObservable3.called, 'iterator closes on value conversion error');
+});
diff --git a/tests/unit-pure/esnext.iterator.join.js b/tests/unit-pure/esnext.iterator.join.js
new file mode 100644
index 000000000000..cb22ee745248
--- /dev/null
+++ b/tests/unit-pure/esnext.iterator.join.js
@@ -0,0 +1,42 @@
+import { createIterator } from '../helpers/helpers.js';
+
+import Iterator from '@core-js/pure/full/iterator';
+import Symbol from '@core-js/pure/es/symbol';
+
+QUnit.test('Iterator#join', assert => {
+  const { join } = Iterator.prototype;
+
+  assert.isFunction(join);
+  assert.arity(join, 1);
+  assert.name(join, 'join');
+  assert.nonEnumerable(Iterator.prototype, 'join');
+
+  const observableReturn = {
+    return() {
+      this.called = true;
+      return { done: true, value: undefined };
+    },
+  };
+
+  const itObservable1 = createIterator([1, 2, 3], observableReturn);
+  assert.strictEqual(join.call(itObservable1), '1,2,3', 'without separator attribute');
+  assert.true(itObservable1.called, 'iterator closes');
+  assert.strictEqual(join.call(createIterator([1, 2, 3]), '-'), '1-2-3', 'with separator attribute');
+  assert.strictEqual(join.call(createIterator([1, null, 3]), '-'), '1--3', 'skips nullish values');
+  assert.strictEqual(join.call(createIterator([1, undefined, 3]), '-'), '1--3', 'skips undefined values');
+  assert.strictEqual(join.call(createIterator([])), '', 'empty iterator');
+
+  assert.throws(() => join.call(''), TypeError, 'iterable non-object this');
+  assert.throws(() => join.call(undefined), TypeError, 'non-iterable-object this #1');
+  assert.throws(() => join.call(null), TypeError, 'non-iterable-object this #2');
+  assert.throws(() => join.call(5), TypeError, 'non-iterable-object this #3');
+
+  const symbol = Symbol('a');
+  const itObservable2 = createIterator([1, 2, 3], observableReturn);
+  assert.throws(() => join.call(itObservable2, symbol), TypeError, 'throws on symbol separator');
+  assert.true(itObservable2.called, 'iterator closes on argument validation error');
+
+  const itObservable3 = createIterator([1, symbol, 3], observableReturn);
+  assert.throws(() => join.call(itObservable3), TypeError, 'throws on non-stringable value');
+  assert.true(itObservable3.called, 'iterator closes on value conversion error');
+});