diff --git a/CHANGES.rst b/CHANGES.rst
index faa3317..8103ff8 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -5,6 +5,7 @@ Changes
 
 Unreleased
 ==========
+- Dependencies: Permitted installation of Sphinx 9.x
 
 2.1.5 - 2025/11/19
 ==================
diff --git a/common-build/requirements.txt b/common-build/requirements.txt
index 5c85889..5329364 100644
--- a/common-build/requirements.txt
+++ b/common-build/requirements.txt
@@ -7,4 +7,4 @@ sphinx-autobuild>=2024
 # These versions should match the Read The Docs environment (please update if
 # you notice this is not the case)
 setuptools>=41.1.0
-sphinx>=3,<9
+sphinx>=3,<10
diff --git a/docs/requirements.txt b/docs/requirements.txt
index a17147c..5a1db45 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -1,2 +1,2 @@
-sphinx>=3,<9
+sphinx>=3,<10
 crate-docs-theme>=0.42.0
diff --git a/helpers/preview/etc/requirements.txt b/helpers/preview/etc/requirements.txt
index c6965f0..a42fe74 100644
--- a/helpers/preview/etc/requirements.txt
+++ b/helpers/preview/etc/requirements.txt
@@ -1,3 +1,3 @@
-sphinx>=3,<6
+sphinx>=3,<10
 sphinx-autobuild
 sphinx-rtd-theme