diff --git a/pkgs/development/interpreters/python/cpython/2.7/default.nix b/pkgs/development/interpreters/python/cpython/2.7/default.nix
index 1503c221bdd..de980f1ca68 100644
--- a/pkgs/development/interpreters/python/cpython/2.7/default.nix
+++ b/pkgs/development/interpreters/python/cpython/2.7/default.nix
@@ -256,6 +256,11 @@ in with passthru; stdenv.mkDerivation ({
 
     inherit passthru;
 
+    postFixup = ''
+      # Include a sitecustomize.py file. Note it causes an error when it's in postInstall with 2.7.
+      cp ${../../sitecustomize.py} $out/${sitePackages}/sitecustomize.py
+    '';
+
     enableParallelBuilding = true;
 
     doCheck = false; # expensive, and fails
diff --git a/pkgs/development/interpreters/python/cpython/default.nix b/pkgs/development/interpreters/python/cpython/default.nix
index 5eb7f3ec166..2f398d8dccb 100644
--- a/pkgs/development/interpreters/python/cpython/default.nix
+++ b/pkgs/development/interpreters/python/cpython/default.nix
@@ -221,6 +221,9 @@ in with passthru; stdenv.mkDerivation {
     find $out/lib/python*/config-* -type f -print -exec nuke-refs -e $out '{}' +
     find $out/lib -name '_sysconfigdata*.py*' -print -exec nuke-refs -e $out '{}' +
 
+    # Include a sitecustomize.py file
+    cp ${../sitecustomize.py} $out/${sitePackages}/sitecustomize.py
+
     # Determinism: rebuild all bytecode
     # We exclude lib2to3 because that's Python 2 code which fails
     # We rebuild three times, once for each optimization level
diff --git a/pkgs/development/interpreters/python/pypy/default.nix b/pkgs/development/interpreters/python/pypy/default.nix
index 42b652978bc..705ddb2a6f5 100644
--- a/pkgs/development/interpreters/python/pypy/default.nix
+++ b/pkgs/development/interpreters/python/pypy/default.nix
@@ -137,6 +137,9 @@ in with passthru; stdenv.mkDerivation rec {
 
     # Python on Nix is not manylinux1 compatible. https://github.com/NixOS/nixpkgs/issues/18484
     echo "manylinux1_compatible=False" >> $out/lib/${libPrefix}/_manylinux.py
+
+    # Include a sitecustomize.py file
+    cp ${../sitecustomize.py} $out/${sitePackages}/sitecustomize.py
   '';
 
   inherit passthru;
diff --git a/pkgs/development/interpreters/python/pypy/prebuilt.nix b/pkgs/development/interpreters/python/pypy/prebuilt.nix
index 6af198e65ea..1ac779ed25e 100644
--- a/pkgs/development/interpreters/python/pypy/prebuilt.nix
+++ b/pkgs/development/interpreters/python/pypy/prebuilt.nix
@@ -84,6 +84,10 @@ in with passthru; stdenv.mkDerivation {
     echo "Removing bytecode"
     find . -name "__pycache__" -type d -depth -exec rm -rf {} \;
     popd
+
+    # Include a sitecustomize.py file
+    cp ${../sitecustomize.py} $out/${sitePackages}/sitecustomize.py
+
   '';
 
   doInstallCheck = true;
diff --git a/pkgs/development/interpreters/python/sitecustomize.py b/pkgs/development/interpreters/python/sitecustomize.py
new file mode 100644
index 00000000000..7e43be56147
--- /dev/null
+++ b/pkgs/development/interpreters/python/sitecustomize.py
@@ -0,0 +1,18 @@
+"""
+This is a Nix-specific module for discovering modules built with Nix.
+
+The module recursively adds paths that are on `NIX_PYTHONPATH` to `sys.path`. In
+order to process possible `.pth` files `site.addsitedir` is used.
+
+The paths listed in `PYTHONPATH` are added to `sys.path` afterwards, but they
+will be added before the entries we add here and thus take precedence.
+
+Note the `NIX_PYTHONPATH` environment variable is unset in order to prevent leakage.
+"""
+import site
+import os
+import functools
+
+paths = os.environ.pop('NIX_PYTHONPATH', None)
+if paths:
+    functools.reduce(lambda k, p: site.addsitedir(p, k), paths.split(':'), site._init_pathinfo())