diff --git a/phpcs.xml b/phpcs.xml
index 004701155..c267cedd8 100644
--- a/phpcs.xml
+++ b/phpcs.xml
@@ -88,6 +88,12 @@
         <exclude name="SlevomatCodingStandard.TypeHints.DeclareStrictTypes.DeclareStrictTypesMissing"/>
     </rule>
 
+    <!--
+        Requires only one namespace in a file.
+        https://github.com/slevomat/coding-standard#slevomatcodingstandardnamespacesrequireonenamespaceinfile
+     -->
+    <rule ref="SlevomatCodingStandard.Namespaces.RequireOneNamespaceInFile"/>
+
     <!--
         Handle imports from namespaces.
         https://github.com/slevomat/coding-standard#slevomatcodingstandardnamespacesreferenceusednamesonly-