diff --git a/projects/postcss/Dockerfile b/projects/postcss/Dockerfile
new file mode 100644
index 000000000000..ffcc6433de55
--- /dev/null
+++ b/projects/postcss/Dockerfile
@@ -0,0 +1,32 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+################################################################################
+
+FROM gcr.io/oss-fuzz-base/base-builder-javascript
+
+COPY build.sh $SRC/
+
+# PostCSS's default branch is `main`. Pin it explicitly so the integration
+# does not silently break if the default ever changes. The fuzzing harness
+# (`test/fuzzing/fuzz_parse.js`) and its dictionary live in this repo, so
+# they are picked up by the clone without a separate COPY step.
+RUN git clone --depth 1 -b main https://github.com/postcss/postcss
+
+# postcss-parser-tests is the upstream-maintained collection of CSS test
+# cases. We use its `cases/` directory as the seed corpus so the fuzzer
+# starts mutating from real, parser-shaped inputs.
+RUN git clone --depth 1 -b main https://github.com/postcss/postcss-parser-tests
+
+WORKDIR $SRC/postcss
diff --git a/projects/postcss/build.sh b/projects/postcss/build.sh
new file mode 100755
index 000000000000..4de4c702aa71
--- /dev/null
+++ b/projects/postcss/build.sh
@@ -0,0 +1,38 @@
+#!/bin/bash -eu
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+################################################################################
+
+# Install runtime dependencies only. PostCSS's devDependencies pull in tools
+# that have peer-dep conflicts and are unrelated to the library's runtime
+# behavior, so we skip them.
+npm install --omit=dev --ignore-scripts --legacy-peer-deps
+npm install --save-dev --legacy-peer-deps @jazzer.js/core
+
+# Build a seed corpus from the upstream postcss-parser-tests CSS cases so
+# the fuzzer starts mutating from realistic, parser-shaped inputs rather
+# than from empty bytes.
+mkdir -p "$WORK/seed_corpus"
+cp "$SRC"/postcss-parser-tests/cases/*.css "$WORK/seed_corpus/"
+(cd "$WORK/seed_corpus" && zip -q -r "$OUT/fuzz_parse_seed_corpus.zip" .)
+
+# Ship the CSS dictionary alongside the fuzzer so libFuzzer can splice in
+# common CSS tokens during mutation. The dictionary lives in the upstream
+# postcss repo under test/fuzzing/, so it is already present in the clone.
+cp "$SRC/postcss/test/fuzzing/fuzz_parse.dict" "$OUT/fuzz_parse.dict"
+
+# Build Fuzzers. The harness lives upstream at test/fuzzing/fuzz_parse.js
+# and is supplied by the postcss clone above.
+compile_javascript_fuzzer postcss test/fuzzing/fuzz_parse.js -i postcss --sync
diff --git a/projects/postcss/fuzz_parse.dict b/projects/postcss/fuzz_parse.dict
new file mode 100644
index 000000000000..210fd1b09163
--- /dev/null
+++ b/projects/postcss/fuzz_parse.dict
@@ -0,0 +1,101 @@
+# Dictionary of common CSS tokens for libFuzzer.
+# Reference: https://www.w3.org/TR/css-syntax-3/
+
+# At-rules
+"@charset "
+"@import "
+"@media "
+"@supports "
+"@font-face "
+"@keyframes "
+"@page "
+"@namespace "
+"@document "
+"@layer "
+"@container "
+"@property "
+"@scope "
+"@counter-style "
+"@font-feature-values "
+
+# Structural punctuation
+"{"
+"}"
+";"
+":"
+","
+"("
+")"
+"["
+"]"
+"/*"
+"*/"
+"!important"
+"--"
+
+# Selectors
+"*"
+">"
+"+"
+"~"
+"::"
+"&"
+
+# Common pseudo-classes / pseudo-elements
+":hover"
+":focus"
+":active"
+":root"
+":not("
+":is("
+":where("
+":has("
+"::before"
+"::after"
+
+# Common properties
+"color:"
+"background:"
+"background-color:"
+"width:"
+"height:"
+"margin:"
+"padding:"
+"border:"
+"display:"
+"position:"
+"font-size:"
+"font-family:"
+"transform:"
+"transition:"
+"animation:"
+"grid-template-columns:"
+"flex:"
+
+# Values / units
+"px"
+"em"
+"rem"
+"%"
+"vh"
+"vw"
+"deg"
+"rgb("
+"rgba("
+"hsl("
+"hsla("
+"calc("
+"var("
+"url("
+"linear-gradient("
+"none"
+"auto"
+"inherit"
+"initial"
+"unset"
+"revert"
+
+# Strings / escapes
+"\""
+"'"
+"\\"
diff --git a/projects/postcss/fuzz_parse.js b/projects/postcss/fuzz_parse.js
new file mode 100644
index 000000000000..47333fcd70e9
--- /dev/null
+++ b/projects/postcss/fuzz_parse.js
@@ -0,0 +1,123 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+////////////////////////////////////////////////////////////////////////////////
+
+const { FuzzedDataProvider } = require('@jazzer.js/core');
+const postcss = require('./lib/postcss');
+
+module.exports.fuzz = function (data) {
+  const provider = new FuzzedDataProvider(data);
+
+  // The CSS input itself is randomized: every byte the fuzzer produces (or
+  // mutates from the seed corpus) flows directly into `cssString` via
+  // consumeRemainingAsString(). The option flags below are read from the
+  // *back* of the buffer (jazzer.js consumes integrals/booleans from the
+  // tail), so seed CSS files from postcss-parser-tests are fed into the
+  // parser nearly verbatim, with only their last few bytes nibbled off as
+  // option control.
+  const useMap = provider.consumeBoolean();
+  const useFrom = provider.consumeBoolean();
+  const useProcessor = provider.consumeBoolean();
+  const splitMode = provider.consumeIntegralInRange(0, 2);
+  const cssString = provider.consumeRemainingAsString();
+
+  const parseOptions = {};
+  if (useFrom) parseOptions.from = 'fuzz.css';
+  if (useMap) parseOptions.map = { inline: false, annotation: false };
+
+  let root;
+  try {
+    root = postcss.parse(cssString, parseOptions);
+  } catch (e) {
+    if (e instanceof postcss.CssSyntaxError) return;
+    throw e;
+  }
+
+  // Walk the AST and exercise common node accessors. This also stresses
+  // raws/source bookkeeping for any node returned by the parser.
+  try {
+    root.walk(node => {
+      void node.type;
+      void node.toString();
+      if (typeof node.error === 'function') {
+        // Generating an error message touches input/source-map machinery.
+        node.error('fuzz').message;
+      }
+    });
+  } catch (e) {
+    if (!isExpected(e, postcss)) throw e;
+  }
+
+  // Round-trip via stringify and re-parse. Output should itself be parseable.
+  let serialized;
+  try {
+    serialized = root.toString();
+  } catch (e) {
+    if (!isExpected(e, postcss)) throw e;
+    return;
+  }
+
+  try {
+    postcss.parse(serialized);
+  } catch (e) {
+    if (!(e instanceof postcss.CssSyntaxError)) throw e;
+  }
+
+  // Exercise the JSON serialization round-trip.
+  try {
+    const json = root.toJSON();
+    postcss.fromJSON(json);
+  } catch (e) {
+    if (!isExpected(e, postcss)) throw e;
+  }
+
+  // Exercise the main public entry point: postcss().process(). This drives
+  // the LazyResult / NoWorkResult pipeline that real plugin chains use.
+  if (useProcessor) {
+    try {
+      const result = postcss().process(cssString, parseOptions);
+      void result.css;
+      void result.warnings();
+    } catch (e) {
+      if (!isExpected(e, postcss)) throw e;
+    }
+  }
+
+  // Exercise the list helpers, which have their own quoting/escape logic.
+  try {
+    if (splitMode === 0) {
+      postcss.list.comma(cssString);
+    } else if (splitMode === 1) {
+      postcss.list.space(cssString);
+    } else {
+      postcss.list.split(cssString, [',', ' '], false);
+    }
+  } catch (e) {
+    if (!isExpected(e, postcss)) throw e;
+  }
+};
+
+function isExpected(error, postcss) {
+  if (error instanceof postcss.CssSyntaxError) return true;
+  if (!error || typeof error.message !== 'string') return false;
+  // Some legitimate inputs reach known-shaped TypeErrors during stringify or
+  // walk because the CSS allows constructs whose textual form is ambiguous.
+  // Suppress only those well-defined cases so real bugs still surface.
+  const benign = [
+    'Unknown node type',
+    'Unknown word',
+  ];
+  return benign.some(msg => error.message.indexOf(msg) !== -1);
+}
diff --git a/projects/postcss/project.yaml b/projects/postcss/project.yaml
new file mode 100644
index 000000000000..512830f271dd
--- /dev/null
+++ b/projects/postcss/project.yaml
@@ -0,0 +1,8 @@
+homepage: https://postcss.org/
+language: javascript
+primary_contact: "andrey@sitnik.es"
+main_repo: https://github.com/postcss/postcss
+fuzzing_engines:
+- libfuzzer
+sanitizers:
+- none