diff --git a/tox.ini b/tox.ini
index 67c18d57..3d138bea 100644
--- a/tox.ini
+++ b/tox.ini
@@ -8,8 +8,12 @@ usedevelop = True
 setenv = OS_STDOUT_NOCAPTURE=False
          OS_STDERR_NOCAPTURE=False
 
+# Nowadays, TOX_CONSTRAINTS_FILE should be used, but some older scripts might
+# still be using UPPER_CONSTRAINTS_FILE, so we check both variables and use the
+# first one that is defined. If none of them is defined, we fallback to the
+# default value.
 deps =
-       -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -c{env:TOX_CONSTRAINTS_FILE:{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = stestr run --slowest {posargs}
@@ -55,8 +59,12 @@ commands =
 
 [testenv:releasenotes]
 basepython = python3
+# Nowadays, TOX_CONSTRAINTS_FILE should be used, but some older scripts might
+# still be using UPPER_CONSTRAINTS_FILE, so we check both variables and use the
+# first one that is defined. If none of them is defined, we fallback to the
+# default value.
 deps =
-  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -c{env:TOX_CONSTRAINTS_FILE:{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}}
   -r{toxinidir}/doc/requirements.txt
 commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html