diff --git a/src/test/java/com/google/devtools/build/lib/rules/python/BUILD b/src/test/java/com/google/devtools/build/lib/rules/python/BUILD
index 8c62fd54a2e6b4..330b2c9ba35c2b 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/python/BUILD
+++ b/src/test/java/com/google/devtools/build/lib/rules/python/BUILD
@@ -150,7 +150,6 @@ java_test(
     name = "PythonVersionSelectTest",
     srcs = ["PythonVersionSelectTest.java"],
     deps = [
-        ":PythonTestUtils",
         "//src/main/java/com/google/devtools/build/lib:build-base",
         "//src/main/java/com/google/devtools/build/lib/actions",
         "//src/main/java/com/google/devtools/build/lib/collect/nestedset",
diff --git a/src/test/java/com/google/devtools/build/lib/rules/python/PythonVersionSelectTest.java b/src/test/java/com/google/devtools/build/lib/rules/python/PythonVersionSelectTest.java
index c9541183bb3d3a..3ff0e537e7e03b 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/python/PythonVersionSelectTest.java
+++ b/src/test/java/com/google/devtools/build/lib/rules/python/PythonVersionSelectTest.java
@@ -15,7 +15,6 @@
 package com.google.devtools.build.lib.rules.python;
 
 import static com.google.common.truth.Truth.assertThat;
-import static com.google.devtools.build.lib.rules.python.PythonTestUtils.ensureDefaultIsPY2;
 
 import com.google.devtools.build.lib.actions.Artifact;
 import com.google.devtools.build.lib.analysis.FileProvider;
@@ -103,8 +102,6 @@ public void cannotSelectOnForcePythonFlagsWithoutOldApi() throws Exception {
    */
   @Test
   public void selectOnPythonVersionTarget() throws Exception {
-    // Expected results assume the default is PY2.
-    ensureDefaultIsPY2();
     // getRuleContext() doesn't populate the information needed to resolve select()s, and
     // ConfiguredTarget doesn't allow us to test an end-to-end view of the behavior of a select().
     // So this test has the select() control srcs and asserts on which one's in the files to build.
@@ -120,11 +117,19 @@ public void selectOnPythonVersionTarget() throws Exception {
         "    }),",
         ")");
 
-    // Default.
-    doTestSelectOnPythonVersionTarget(py2);
+    // Neither --python_version nor --force_python, use default value.
+    doTestSelectOnPythonVersionTarget(py2, "--incompatible_py3_is_default=false");
+    doTestSelectOnPythonVersionTarget(
+        py3,
+        "--incompatible_py3_is_default=true",
+        // PythonConfiguration has a validation check requiring that the new transition semantics be
+        // enabled before we're allowed to set the default to PY3.
+        "--incompatible_allow_python_version_transitions=true");
+
     // No --python_version, trust --force_python.
     doTestSelectOnPythonVersionTarget(py2, "--force_python=PY2");
     doTestSelectOnPythonVersionTarget(py3, "--force_python=PY3");
+
     // --python_version overrides --force_python.
     doTestSelectOnPythonVersionTarget(py2, "--python_version=PY2");
     doTestSelectOnPythonVersionTarget(py2, "--python_version=PY2", "--force_python=PY2");
diff --git a/tools/python/python_version.bzl b/tools/python/python_version.bzl
index 05260e124b4f74..8bb45b86c0dce4 100644
--- a/tools/python/python_version.bzl
+++ b/tools/python/python_version.bzl
@@ -20,9 +20,6 @@ _TRUE = "TRUE"
 _PY2 = "PY2"
 _PY3 = "PY3"
 
-# Keep in sync with PythonVersion.DEFAULT_TARGET_VALUE.
-_DEFAULT_PYTHON_VERSION = "PY2"
-
 def _python_version_flag_impl(ctx):
     # Version is determined using the same logic as in PythonOptions#getPythonVersion:
     #
@@ -34,7 +31,7 @@ def _python_version_flag_impl(ctx):
     elif ctx.attr.force_python_flag != _UNSET:
         version = ctx.attr.force_python_flag
     else:
-        version = _DEFAULT_PYTHON_VERSION
+        version = _PY3 if ctx.attr.incompatible_py3_is_default_flag else _PY2
 
     if version not in ["PY2", "PY3"]:
         fail("Internal error: _python_version_flag should only be able to " +
@@ -46,6 +43,7 @@ _python_version_flag = rule(
     attrs = {
         "force_python_flag": attr.string(mandatory = True, values = [_PY2, _PY3, _UNSET]),
         "python_version_flag": attr.string(mandatory = True, values = [_PY2, _PY3, _UNSET]),
+        "incompatible_py3_is_default_flag": attr.bool(mandatory = True),
     },
 )
 
@@ -60,7 +58,7 @@ def define_python_version_flag(name):
     """
 
     # Config settings for the underlying native flags we depend on:
-    # --force_python and --python_version.
+    # --force_python, --python_version, and --incompatible_py3_is_default.
     native.config_setting(
         name = "_force_python_setting_PY2",
         values = {"force_python": "PY2"},
@@ -81,6 +79,16 @@ def define_python_version_flag(name):
         values = {"python_version": "PY3"},
         visibility = ["//visibility:private"],
     )
+    native.config_setting(
+        name = "_incompatible_py3_is_default_setting_false",
+        values = {"incompatible_py3_is_default": "false"},
+        visibility = ["//visibility:private"],
+    )
+    native.config_setting(
+        name = "_incompatible_py3_is_default_setting_true",
+        values = {"incompatible_py3_is_default": "true"},
+        visibility = ["//visibility:private"],
+    )
 
     _python_version_flag(
         name = name,
@@ -94,5 +102,9 @@ def define_python_version_flag(name):
             ":_python_version_setting_PY3": _PY3,
             "//conditions:default": _UNSET,
         }),
+        incompatible_py3_is_default_flag = select({
+            ":_incompatible_py3_is_default_setting_false": False,
+            ":_incompatible_py3_is_default_setting_true": True,
+        }),
         visibility = ["//visibility:public"],
     )