diff --git a/README.md b/README.md
index 81369da..c0d1c3b 100644
--- a/README.md
+++ b/README.md
@@ -36,6 +36,7 @@ There are three kinds of version labels you can use.
 - `version` - Version of cache to load. Each version will have its own cache. Note, all characters except spaces are allowed.
 - `execute_install_scripts` - Execute Debian package pre and post install script upon restore. See [Caveats / Non-file Dependencies](#non-file-dependencies) for more information.
 - `empty_packages_behavior` - Desired behavior when the given `packages` is empty. `'error'` (default), `'warn'` or `'ignore'`.
+- `add-repository` - Space delimited list of repositories to add via `apt-add-repository` before installing packages. Supports PPA (e.g., `ppa:user/repo`) and other repository formats.
 
 ### Outputs
 
@@ -90,6 +91,36 @@ install_doxygen_deps:
         version: 1.0
 ```
 
+### Using with Third-party PPAs
+
+This example shows how to install packages from a third-party PPA:
+
+```yaml
+install_from_ppa:
+  runs-on: ubuntu-latest
+  steps:
+    - uses: actions/checkout@v4
+    - uses: awalsh128/cache-apt-pkgs-action@latest
+      with:
+        packages: chromium-browser
+        add-repository: ppa:canonical-chromium-builds/stage
+        version: 1.0
+```
+
+You can also add multiple repositories:
+
+```yaml
+install_from_multiple_repos:
+  runs-on: ubuntu-latest
+  steps:
+    - uses: actions/checkout@v4
+    - uses: awalsh128/cache-apt-pkgs-action@latest
+      with:
+        packages: package1 package2
+        add-repository: ppa:user/repo1 ppa:user/repo2
+        version: 1.0
+```
+
 ## Caveats
 
 ### Non-file Dependencies
diff --git a/action.yml b/action.yml
index 81f6870..3c9c6bb 100644
--- a/action.yml
+++ b/action.yml
@@ -35,6 +35,10 @@ inputs:
     description: 'Enable debugging when there are issues with action. Minor performance penalty.'
     required: false
     default: 'false'
+  add-repository:
+    description: 'Space delimited list of repositories to add via apt-add-repository before installing packages. Supports PPA (ppa:user/repo) and other repository formats.'
+    required: false
+    default: ''
 
 outputs:
   cache-hit:
@@ -59,6 +63,7 @@ runs:
           "$VERSION" \
           "$EXEC_INSTALL_SCRIPTS" \
           "$DEBUG" \
+          "$ADD_REPOSITORY" \
           "$PACKAGES"
         if [ -f ~/cache-apt-pkgs/cache_key.md5 ]; then
           echo "CACHE_KEY=$(cat ~/cache-apt-pkgs/cache_key.md5)" >> $GITHUB_ENV
@@ -71,6 +76,7 @@ runs:
          EXEC_INSTALL_SCRIPTS: "${{ inputs.execute_install_scripts }}"
          EMPTY_PACKAGES_BEHAVIOR: "${{ inputs.empty_packages_behavior }}"
          DEBUG: "${{ inputs.debug }}"
+         ADD_REPOSITORY: "${{ inputs.add-repository }}"
          PACKAGES: "${{ inputs.packages }}"
 
     - id: load-cache
@@ -89,6 +95,7 @@ runs:
           "$CACHE_HIT" \
           "$EXEC_INSTALL_SCRIPTS" \
           "$DEBUG" \
+          "$ADD_REPOSITORY" \
           "$PACKAGES"
         function create_list { local list=$(cat ~/cache-apt-pkgs/manifest_${1}.log | tr '\n' ','); echo ${list:0:-1}; };
         echo "package-version-list=$(create_list main)" >> $GITHUB_OUTPUT
@@ -98,6 +105,7 @@ runs:
          CACHE_HIT: "${{ steps.load-cache.outputs.cache-hit }}"
          EXEC_INSTALL_SCRIPTS: "${{ inputs.execute_install_scripts }}"
          DEBUG: "${{ inputs.debug }}"
+         ADD_REPOSITORY: "${{ inputs.add-repository }}"
          PACKAGES: "${{ inputs.packages }}"
 
     - id: upload-logs
diff --git a/install_and_cache_pkgs.sh b/install_and_cache_pkgs.sh
index ce96652..1a544ad 100755
--- a/install_and_cache_pkgs.sh
+++ b/install_and_cache_pkgs.sh
@@ -15,8 +15,11 @@ source "${script_dir}/lib.sh"
 # Directory that holds the cached packages.
 cache_dir="${1}"
 
+# Repositories to add before installing packages.
+add_repository="${3}"
+
 # List of the packages to use.
-input_packages="${@:3}"
+input_packages="${@:4}"
 
 if ! apt-fast --version > /dev/null 2>&1; then
   log "Installing apt-fast for optimized installs..."
@@ -27,6 +30,17 @@ if ! apt-fast --version > /dev/null 2>&1; then
   log_empty_line
 fi
 
+# Add custom repositories if specified
+if [ -n "${add_repository}" ]; then
+  log "Adding custom repositories..."
+  for repository in ${add_repository}; do
+    log "- Adding repository: ${repository}"
+    sudo apt-add-repository -y "${repository}"
+  done
+  log "done"
+  log_empty_line
+fi
+
 log "Updating APT package list..."
 if [[ -z "$(find -H /var/lib/apt/lists -maxdepth 0 -mmin -5)" ]]; then
   sudo apt-fast update > /dev/null
diff --git a/post_cache_action.sh b/post_cache_action.sh
index f71c336..a6a5689 100755
--- a/post_cache_action.sh
+++ b/post_cache_action.sh
@@ -25,13 +25,16 @@ execute_install_scripts="${4}"
 debug="${5}"
 test "${debug}" = "true" && set -x
 
+# Repositories to add before installing packages.
+add_repository="${6}"
+
 # List of the packages to use.
-packages="${@:6}"
+packages="${@:7}"
 
 if test "${cache_hit}" = "true"; then
   ${script_dir}/restore_pkgs.sh "${cache_dir}" "${cache_restore_root}" "${execute_install_scripts}" "${debug}"
 else
-  ${script_dir}/install_and_cache_pkgs.sh "${cache_dir}" "${debug}" ${packages}
+  ${script_dir}/install_and_cache_pkgs.sh "${cache_dir}" "${debug}" "${add_repository}" ${packages}
 fi
 
 log_empty_line
diff --git a/pre_cache_action.sh b/pre_cache_action.sh
index 1ed9f5c..5cb64cb 100755
--- a/pre_cache_action.sh
+++ b/pre_cache_action.sh
@@ -24,8 +24,11 @@ execute_install_scripts="${3}"
 # Debug mode for diagnosing issues.
 debug="${4}"
 
+# Repositories to add before installing packages.
+add_repository="${5}"
+
 # List of the packages to use.
-input_packages="${@:5}"
+input_packages="${@:6}"
 
 # Trim commas, excess spaces, and sort.
 log "Normalizing package list..."
@@ -62,6 +65,21 @@ fi
 
 validate_bool "${execute_install_scripts}" execute_install_scripts 4
 
+# Basic validation for repository parameter
+if [ -n "${add_repository}" ]; then
+  log "Validating repository parameter..."
+  for repository in ${add_repository}; do
+    # Check if repository format looks valid (basic check)
+    if [[ "${repository}" =~ [^a-zA-Z0-9:\/.-] ]]; then
+      log "aborted"
+      log "Repository '${repository}' contains invalid characters." >&2
+      log "Supported formats: 'ppa:user/repo', 'deb http://...', 'http://...', 'multiverse', etc." >&2
+      exit 6
+    fi
+  done
+  log "done"
+fi
+
 log "done"
 
 log_empty_line
@@ -81,6 +99,12 @@ log "- CPU architecture is '${cpu_arch}'."
 
 value="${packages} @ ${version} ${force_update_inc}"
 
+# Include repositories in cache key to ensure different repos get different caches
+if [ -n "${add_repository}" ]; then
+  value="${value} ${add_repository}"
+  log "- Repositories '${add_repository}' added to value."
+fi
+
 # Don't invalidate existing caches for the standard Ubuntu runners
 if [ "${cpu_arch}" != "x86_64" ]; then
   value="${value} ${cpu_arch}"
diff --git a/restore_pkgs.sh b/restore_pkgs.sh
index 418f9f0..4556265 100755
--- a/restore_pkgs.sh
+++ b/restore_pkgs.sh
@@ -40,7 +40,7 @@ log "done"
 log_empty_line
 
 # Only search for archived results. Manifest and cache key also live here.
-cached_filepaths=$(ls -1 "${cache_dir}"/*.tar | sort)
+cached_filepaths=$(ls -1 "${cache_dir}"/*.tar 2>/dev/null | sort)
 cached_filecount=$(echo ${cached_filepaths} | wc -w)
 
 log "Restoring ${cached_filecount} packages from cache..."