diff --git a/third_party/nix/BUILD b/third_party/nix/BUILD
new file mode 100644
index 0000000..c0821e6
--- /dev/null
+++ b/third_party/nix/BUILD
@@ -0,0 +1,21 @@
+load("@rules_python//python:defs.bzl", "py_runtime_pair")
+
+# Python3 toolchain definition that uses //third_party/nix:python.nix (via
+# external repository).
+
+py_runtime(
+    name = "py3_runtime",
+    interpreter = "@hscloud_nix_python3//:python3",
+    python_version = "PY3",
+)
+
+py_runtime_pair(
+    name = "py_runtime_pair",
+    py3_runtime = ":py3_runtime",
+)
+
+toolchain(
+    name = "py_toolchain",
+    toolchain = ":py_runtime_pair",
+    toolchain_type = "@rules_python//python:toolchain_type",
+)