diff --git a/.release-notes/protect-ignore-file-size.md b/.release-notes/protect-ignore-file-size.md
new file mode 100644
index 0000000000..6293584654
--- /dev/null
+++ b/.release-notes/protect-ignore-file-size.md
@@ -0,0 +1,3 @@
+## Protect pony-lint against oversize ignore files
+
+pony-lint now rejects `.gitignore` and `.ignore` files larger than 64 KB. With hierarchical ignore loading, each directory in a project can have its own ignore files — an unexpectedly large file could cause excessive memory consumption. Ignore files that exceed the limit or that cannot be opened produce a `lint/ignore-error` diagnostic with exit code 2.
diff --git a/tools/pony-lint/ignore_matcher.pony b/tools/pony-lint/ignore_matcher.pony
index 8ddbfd7870..d6c863c3c3 100644
--- a/tools/pony-lint/ignore_matcher.pony
+++ b/tools/pony-lint/ignore_matcher.pony
@@ -46,6 +46,7 @@ class ref IgnoreMatcher
   let _root: String val
   let _in_git_repo: Bool
   let _rules: Array[(IgnorePattern val, String val)]
+  let _errors: Array[(String val, String val)]
 
   new ref create(file_auth: FileAuth, root: (String val | None)) =>
     """
@@ -63,6 +64,7 @@ class ref IgnoreMatcher
       _in_git_repo = false
     end
     _rules = Array[(IgnorePattern val, String val)]
+    _errors = Array[(String val, String val)]
 
   fun ref load_directory(dir_path: String val) =>
     """
@@ -75,15 +77,47 @@ class ref IgnoreMatcher
     end
     _load_file(Path.join(dir_path, ".ignore"), dir_path)
 
+  fun errors(): this->Array[(String val, String val)] =>
+    """
+    Return errors accumulated during `load_directory` calls. Each entry is
+    a `(message, file_path)` tuple. Call `clear_errors` after draining.
+    """
+    _errors
+
+  fun ref clear_errors() =>
+    """
+    Remove all accumulated errors. Call after draining `errors()`.
+    """
+    _errors.clear()
+
   fun ref _load_file(file_path: String val, base_dir: String val) =>
     """
     Parse all lines from an ignore file and append the resulting rules.
+
+    Rejects files that cannot be opened or are larger than 64 KB to prevent
+    unexpected memory consumption, especially with hierarchical ignore
+    loading where each directory can have its own `.gitignore` and `.ignore`
+    files.
     """
     let fp = FilePath(_file_auth, file_path)
     if not fp.exists() then return end
     let file = File.open(fp)
-    if not file.valid() then return end
-    let content: String val = file.read_string(file.size())
+    if not file.valid() then
+      _errors.push((
+        "could not open ignore file: " + file_path,
+        file_path))
+      return
+    end
+    let size = file.size()
+    if size > _max_ignore_file_size() then
+      file.dispose()
+      _errors.push((
+        "ignore file too large (" + size.string() + " bytes, max "
+          + _max_ignore_file_size().string() + "): " + file_path,
+        file_path))
+      return
+    end
+    let content: String val = file.read_string(size)
     file.dispose()
     // Normalize line endings and parse
     let clean_content: String val =
@@ -168,3 +202,5 @@ class ref IgnoreMatcher
     else
       false
     end
+
+  fun _max_ignore_file_size(): USize => 65_536
diff --git a/tools/pony-lint/linter.pony b/tools/pony-lint/linter.pony
index f8ca9247e8..43deb416a9 100644
--- a/tools/pony-lint/linter.pony
+++ b/tools/pony-lint/linter.pony
@@ -480,6 +480,17 @@ class val Linter
       end
     end
 
+    // Surface errors from intermediate ignore file loading
+    for (msg, path) in matcher.errors().values() do
+      config_errors.push(Diagnostic(
+        "lint/ignore-error",
+        msg,
+        try Path.rel(_cwd, path)? else path end,
+        0,
+        0))
+    end
+    matcher.clear_errors()
+
     // Pre-load subdirectory configs from hierarchy root through intermediate
     // directories to each target. Uses _root_dir (config hierarchy root),
     // not the git root.
@@ -674,6 +685,17 @@ class ref _FileCollector is WalkHandler
     // Load ignore files for this directory
     _matcher.load_directory(dir_path.path)
 
+    // Surface errors from ignore file loading
+    for (msg, path) in _matcher.errors().values() do
+      _config_errors.push(Diagnostic(
+        "lint/ignore-error",
+        msg,
+        try Path.rel(_cwd, path)? else path end,
+        0,
+        0))
+    end
+    _matcher.clear_errors()
+
     // Load subdirectory config if present
     _load_config(dir_path.path)
 
diff --git a/tools/pony-lint/test/_test_ignore.pony b/tools/pony-lint/test/_test_ignore.pony
index f73479a1a9..4a648e9afa 100644
--- a/tools/pony-lint/test/_test_ignore.pony
+++ b/tools/pony-lint/test/_test_ignore.pony
@@ -424,6 +424,51 @@ class \nodoc\ _TestIgnoreMatcherHierarchical is UnitTest
       h.fail("could not create temp directory")
     end
 
+class \nodoc\ _TestIgnoreMatcherOversizedFile is UnitTest
+  """Oversized ignore file produces an error and loads no rules."""
+  fun name(): String => "IgnoreMatcher: oversized file -> error"
+
+  fun apply(h: TestHelper) =>
+    let auth = h.env.root
+    try
+      let tmp = FilePath.mkdtemp(FileAuth(auth), "ignore-test")?
+      let git_dir =
+        FilePath(FileAuth(auth), Path.join(tmp.path, ".git"))
+      git_dir.mkdir()
+      // Write a .gitignore that exceeds the 64 KB limit
+      let gitignore_path =
+        Path.join(tmp.path, ".gitignore")
+      let gitignore =
+        File(FilePath(FileAuth(auth), gitignore_path))
+      let content = recover val String(65_537) .> append("x" * 65_537) end
+      gitignore.print(content)
+      gitignore.dispose()
+
+      let matcher = lint.IgnoreMatcher(FileAuth(auth), tmp.path)
+      matcher.load_directory(tmp.path)
+
+      // Should not have loaded any rules
+      h.assert_false(
+        matcher.is_ignored(
+          Path.join(tmp.path, "anything"), "anything", false))
+
+      // Should have recorded an error
+      let errs = matcher.errors()
+      h.assert_eq[USize](1, errs.size())
+      try
+        (let msg, _) = errs(0)?
+        h.assert_true(msg.contains("too large"))
+      else
+        h.fail("could not read error")
+      end
+
+      FilePath(FileAuth(auth), gitignore_path).remove()
+      git_dir.remove()
+      tmp.remove()
+    else
+      h.fail("could not create temp directory")
+    end
+
 class \nodoc\ _TestIgnoreMatcherAnchoredPattern is UnitTest
   """Anchored patterns match against the relative path from base_dir."""
   fun name(): String => "IgnoreMatcher: anchored pattern"
diff --git a/tools/pony-lint/test/main.pony b/tools/pony-lint/test/main.pony
index de50329fa2..0d412bc5a5 100644
--- a/tools/pony-lint/test/main.pony
+++ b/tools/pony-lint/test/main.pony
@@ -169,6 +169,7 @@ actor \nodoc\ Main is TestList
     test(_TestIgnoreMatcherNonGitIgnoresGitignore)
     test(_TestIgnoreMatcherHierarchical)
     test(_TestIgnoreMatcherAnchoredPattern)
+    test(_TestIgnoreMatcherOversizedFile)
 
     // Linter tests
     test(_TestLinterSingleFile)