diff --git a/.github/workflows/build.yaml b/.github/workflows/build.yaml
index 3d6b1be0..79fbfb35 100644
--- a/.github/workflows/build.yaml
+++ b/.github/workflows/build.yaml
@@ -75,6 +75,10 @@ jobs:
         if: ${{ steps.params.outputs.major < 8 && matrix.archs.arch == 'x86_64-linux' }}
         run: nix-build -A outputs.checks.${{ matrix.archs.arch }}.${{ steps.params.outputs.attr }}-redis3
 
+      - name: Build Mcrypt extension
+        if: ${{ steps.params.outputs.major < 7 }}
+        run: nix-build -A outputs.checks.${{ matrix.archs.arch }}.${{ steps.params.outputs.attr }}-mcrypt
+
       - name: Build MySQL extension
         if: ${{ steps.params.outputs.major < 7 }}
         run: nix-build -A outputs.checks.${{ matrix.archs.arch }}.${{ steps.params.outputs.attr }}-mysql
diff --git a/checks.nix b/checks.nix
index b322f85d..96323ad0 100644
--- a/checks.nix
+++ b/checks.nix
@@ -13,6 +13,12 @@ let
       drv = { php, ... }: php;
     };
 
+    mcrypt = {
+      description = "Build Mcrypt extension";
+      enabled = { php, lib, ... }: lib.versionOlder php.version "7";
+      drv = { php, ... }: php.extensions.mcrypt;
+    };
+
     imagick = {
       description = "Build Imagick extension";
       drv = { php, ... }: php.extensions.imagick;
diff --git a/pkgs/package-overrides.nix b/pkgs/package-overrides.nix
index c59a8136..27c0ab77 100644
--- a/pkgs/package-overrides.nix
+++ b/pkgs/package-overrides.nix
@@ -398,6 +398,15 @@ in
       else
         prev.extensions.mbstring;
 
+    mcrypt =
+      if lib.versionOlder prev.php.version "7.0" then
+        prev.mkExtension {
+          name = "mcrypt";
+          configureFlags = [ "--with-mcrypt=${pkgs.libmcrypt.outPath}" ];
+        }
+      else
+        throw "php.extensions.mcrypt requires PHP version < 7.0.";
+
     memcached =
       if lib.versionOlder prev.php.version "7.0" then
         prev.extensions.memcached.overrideAttrs (attrs: {