diff --git a/.github/workflows/readme-package-check.yml b/.github/workflows/readme-package-check.yml
new file mode 100644
index 00000000..ae437c42
--- /dev/null
+++ b/.github/workflows/readme-package-check.yml
@@ -0,0 +1,30 @@
+name: README Package Catalog Check
+
+on:
+  pull_request:
+    paths:
+      - 'packages/**/composer.json'
+      - 'packages/*'
+      - 'README.md'
+      - 'bin/check-readme-packages.sh'
+      - '.github/workflows/readme-package-check.yml'
+  push:
+    branches:
+      - develop
+      - main
+    paths:
+      - 'packages/**/composer.json'
+      - 'README.md'
+      - 'bin/check-readme-packages.sh'
+      - '.github/workflows/readme-package-check.yml'
+
+jobs:
+  check:
+    name: Catalog drift check
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Run drift check
+        run: bin/check-readme-packages.sh
diff --git a/bin/check-readme-packages.sh b/bin/check-readme-packages.sh
new file mode 100755
index 00000000..38225938
--- /dev/null
+++ b/bin/check-readme-packages.sh
@@ -0,0 +1,103 @@
+#!/usr/bin/env bash
+#
+# check-readme-packages.sh
+#
+# Asserts that the root README.md package catalog is in sync with the
+# `packages/` directory. Every Composer package with `type: marko-module`
+# under `packages/` must have a row in the catalog, and every catalog row
+# must reference a real package directory.
+#
+# Exits 0 on success. Exits 1 with a human-readable diff on drift.
+#
+# Run locally:   bin/check-readme-packages.sh
+# Run in CI:     same — invoked from .github/workflows/readme-package-check.yml
+#
+set -euo pipefail
+
+REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+cd "$REPO_ROOT"
+
+README="README.md"
+PACKAGES_DIR="packages"
+
+if [[ ! -f "$README" ]]; then
+    echo "Error: $README not found at repo root." >&2
+    exit 2
+fi
+
+if [[ ! -d "$PACKAGES_DIR" ]]; then
+    echo "Error: $PACKAGES_DIR/ directory not found at repo root." >&2
+    exit 2
+fi
+
+if ! command -v jq >/dev/null 2>&1; then
+    echo "Error: jq is required. Install with: brew install jq" >&2
+    exit 2
+fi
+
+# Collect every package basename under packages/ that is not a project template.
+# `type: project` (currently just marko/skeleton) is the only kind that legitimately
+# stays out of the README catalog — everything else is a module of some flavor
+# (type: library, marko-module, metapackage, or unset).
+mapfile -t MODULE_PKGS < <(
+    for composer in "$PACKAGES_DIR"/*/composer.json; do
+        [[ -f "$composer" ]] || continue
+        type=$(jq -r '.type // ""' "$composer")
+        if [[ "$type" != "project" ]]; then
+            basename "$(dirname "$composer")"
+        fi
+    done | sort -u
+)
+
+# Scrape catalog entries from README. Matches `packages/<name>/README.md` links.
+mapfile -t README_PKGS < <(
+    grep -oE 'packages/[a-z0-9][a-z0-9-]*/README\.md' "$README" \
+        | sed -E 's|packages/([^/]+)/README\.md|\1|' \
+        | sort -u
+)
+
+# Diff both directions.
+missing_from_readme=()
+for pkg in "${MODULE_PKGS[@]}"; do
+    if ! printf '%s\n' "${README_PKGS[@]}" | grep -qxF "$pkg"; then
+        missing_from_readme+=("$pkg")
+    fi
+done
+
+stale_in_readme=()
+for pkg in "${README_PKGS[@]}"; do
+    if ! printf '%s\n' "${MODULE_PKGS[@]}" | grep -qxF "$pkg"; then
+        stale_in_readme+=("$pkg")
+    fi
+done
+
+if [[ ${#missing_from_readme[@]} -eq 0 && ${#stale_in_readme[@]} -eq 0 ]]; then
+    echo "README.md package catalog is aligned with packages/ (${#MODULE_PKGS[@]} modules)."
+    exit 0
+fi
+
+echo "README.md package catalog is out of sync with packages/." >&2
+echo >&2
+
+if [[ ${#missing_from_readme[@]} -gt 0 ]]; then
+    echo "Packages present under packages/ but missing from README.md:" >&2
+    for pkg in "${missing_from_readme[@]}"; do
+        echo "  - $pkg" >&2
+    done
+    echo >&2
+    echo "  Fix: add a row to the appropriate section of README.md, e.g." >&2
+    echo "       | [${missing_from_readme[0]}](packages/${missing_from_readme[0]}/README.md) | <description> |" >&2
+    echo >&2
+fi
+
+if [[ ${#stale_in_readme[@]} -gt 0 ]]; then
+    echo "Packages referenced in README.md but not present under packages/ (or type: project):" >&2
+    for pkg in "${stale_in_readme[@]}"; do
+        echo "  - $pkg" >&2
+    done
+    echo >&2
+    echo "  Fix: remove the stale row from README.md, or restore the package." >&2
+    echo >&2
+fi
+
+exit 1