diff --git a/python/Makefile b/python/Makefile
index 470d32b9d4..6c7e1ab32c 100644
--- a/python/Makefile
+++ b/python/Makefile
@@ -8,7 +8,7 @@ debug = not-set
 ifneq ($(python), not-set)
     PYTHON=$(python)
 else
-    PYTHON=python
+    PYTHON=python3
 endif
 
 ifneq ($(debug), not-set)
diff --git a/python/README.md b/python/README.md
index c1cd00e4aa..57a3179bac 100644
--- a/python/README.md
+++ b/python/README.md
@@ -14,7 +14,7 @@ thing, but support different options:
 
    You can specify a few arguments to this:
    
-   - python version: `make python=python3 test`
+   - python version: `make python=python3.9 test` (defaults to `python3`; you should expect errors if trying to run with python2)
    - verbose: `make verbose=true test`
    - debug: `make debug=true test`
 
diff --git a/python/ctsm/test/joblauncher/test_unit_job_launcher_no_batch.py b/python/ctsm/test/joblauncher/test_unit_job_launcher_no_batch.py
old mode 100644
new mode 100755
index c407c62904..53bb6dc07d
--- a/python/ctsm/test/joblauncher/test_unit_job_launcher_no_batch.py
+++ b/python/ctsm/test/joblauncher/test_unit_job_launcher_no_batch.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Unit tests for job_launcher_no_batch
 """
diff --git a/python/ctsm/test/test_sys_lilac_build_ctsm.py b/python/ctsm/test/test_sys_lilac_build_ctsm.py
index 74121ba90d..5a44688171 100755
--- a/python/ctsm/test/test_sys_lilac_build_ctsm.py
+++ b/python/ctsm/test/test_sys_lilac_build_ctsm.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """System tests for lilac_build_ctsm
 
diff --git a/python/ctsm/test/test_unit_lilac_build_ctsm.py b/python/ctsm/test/test_unit_lilac_build_ctsm.py
index 677de63da4..3c1a600326 100755
--- a/python/ctsm/test/test_unit_lilac_build_ctsm.py
+++ b/python/ctsm/test/test_unit_lilac_build_ctsm.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Unit tests for lilac_build_ctsm
 """
diff --git a/python/ctsm/test/test_unit_lilac_make_runtime_inputs.py b/python/ctsm/test/test_unit_lilac_make_runtime_inputs.py
index 7c94089269..e6b602b3d7 100755
--- a/python/ctsm/test/test_unit_lilac_make_runtime_inputs.py
+++ b/python/ctsm/test/test_unit_lilac_make_runtime_inputs.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Unit tests for lilac_make_runtime_inputs
 """
diff --git a/python/ctsm/test/test_unit_machine.py b/python/ctsm/test/test_unit_machine.py
index 65cd73620e..6a2f7ac172 100755
--- a/python/ctsm/test/test_unit_machine.py
+++ b/python/ctsm/test/test_unit_machine.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Unit tests for machine
 """
diff --git a/python/ctsm/test/test_unit_path_utils.py b/python/ctsm/test/test_unit_path_utils.py
index 9d4d1a78ff..9fc996aa2c 100755
--- a/python/ctsm/test/test_unit_path_utils.py
+++ b/python/ctsm/test/test_unit_path_utils.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Unit tests for path_utils
 """
diff --git a/python/ctsm/test/test_unit_run_sys_tests.py b/python/ctsm/test/test_unit_run_sys_tests.py
index 78ab02a648..8a53081a5b 100755
--- a/python/ctsm/test/test_unit_run_sys_tests.py
+++ b/python/ctsm/test/test_unit_run_sys_tests.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Unit tests for run_sys_tests
 """
diff --git a/python/ctsm/test/test_unit_utils.py b/python/ctsm/test/test_unit_utils.py
index 34449aa93c..4a3fbbbb15 100755
--- a/python/ctsm/test/test_unit_utils.py
+++ b/python/ctsm/test/test_unit_utils.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Unit tests for utils
 """
diff --git a/python/run_ctsm_py_tests b/python/run_ctsm_py_tests
index ef56f74740..a3da6fdb1f 100755
--- a/python/run_ctsm_py_tests
+++ b/python/run_ctsm_py_tests
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 """Driver for running the unit tests of the python code
 
 We use this rather than simply relying on 'python -m unittest discover' so we can do some
diff --git a/run_sys_tests b/run_sys_tests
index bccf6f00e1..48e6c71370 100755
--- a/run_sys_tests
+++ b/run_sys_tests
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 """Driver for running CTSM system tests"""
 
 import os