diff --git a/change_notes/2026-08-24-fix-fp-rule-5-10-1-hash-specialization-reserved-namespace.md b/change_notes/2026-08-24-fix-fp-rule-5-10-1-hash-specialization-reserved-namespace.md
new file mode 100644
index 000000000..d50c5ea90
--- /dev/null
+++ b/change_notes/2026-08-24-fix-fp-rule-5-10-1-hash-specialization-reserved-namespace.md
@@ -0,0 +1,5 @@
+- `RULE-5-10-1` - `PoorlyFormedIdentifier.ql`:
+  - Fixed false positives where a local variable or function parameter was reported as
+    "defined in reserved namespace" merely because its enclosing function is the body of
+    an explicit template specialization that C++ permits users to add to namespace `std`
+    (for example, `std::hash<UserType>::operator()`'s parameter and local names).
diff --git a/cpp/common/src/codingstandards/cpp/Identifiers.qll b/cpp/common/src/codingstandards/cpp/Identifiers.qll
index 23751ac26..cbacb9b8e 100644
--- a/cpp/common/src/codingstandards/cpp/Identifiers.qll
+++ b/cpp/common/src/codingstandards/cpp/Identifiers.qll
@@ -361,7 +361,20 @@ private module IdentifierIntroductionImpl {
 
     override string getAnIdent() { result = this.getName() }
 
-    override Namespace getNamespace() { result = variable.getNamespace() }
+    override Namespace getNamespace() {
+      // A parameter or local variable is scoped to the body of its enclosing function, not to
+      // that function's enclosing namespace. It does not itself become a new member of the
+      // namespace, so it cannot "pollute" a reserved namespace such as `std`, regardless of which
+      // namespace its enclosing function happens to be declared in. This matters in particular
+      // for the bodies of explicit template specializations that C++ explicitly permits users to
+      // add to namespace `std` (such as `std::hash<UserType>`), where parameter and local names
+      // are otherwise ordinary user-chosen identifiers that merely happen to be lexically nested
+      // inside `namespace std { ... }`. The function or class that actually introduces the
+      // specialization's name is still checked against `isReservedNamespace` independently, via
+      // `FunctionDeclarationEntryIdentifier` / `TypeDeclarationEntryIdentifier`.
+      not variable instanceof LocalScopeVariable and
+      result = variable.getNamespace()
+    }
   }
 
   /**
diff --git a/cpp/misra/test/rules/RULE-5-10-1/PoorlyFormedIdentifier.expected b/cpp/misra/test/rules/RULE-5-10-1/PoorlyFormedIdentifier.expected
index 1c5abc431..ad045a88a 100644
--- a/cpp/misra/test/rules/RULE-5-10-1/PoorlyFormedIdentifier.expected
+++ b/cpp/misra/test/rules/RULE-5-10-1/PoorlyFormedIdentifier.expected
@@ -51,3 +51,4 @@
 | test.cpp:186:7:186:12 | wint_t | Identifier 'wint_t' is a reserved name. |
 | test.cpp:203:1:203:42 | __PRETTY_FUNCTION__ | Identifier '__PRETTY_FUNCTION__' contains double underscores. |
 | test.cpp:203:1:203:42 | __PRETTY_FUNCTION__ | Identifier '__PRETTY_FUNCTION__' starts with underscore. |
+| test.cpp:263:6:263:21 | new_std_function | Identifier 'new_std_function' is defined in reserved namespace. |
diff --git a/cpp/misra/test/rules/RULE-5-10-1/test.cpp b/cpp/misra/test/rules/RULE-5-10-1/test.cpp
index b529a971c..3f74e104c 100644
--- a/cpp/misra/test/rules/RULE-5-10-1/test.cpp
+++ b/cpp/misra/test/rules/RULE-5-10-1/test.cpp
@@ -222,4 +222,43 @@ struct hash<int> { // COMPLIANT - rule does not apply to template
   std::size_t operator()(const int &x) const {
     return static_cast<std::size_t>(x);
   }
-};
\ No newline at end of file
+};
+
+// Test case for RULE-5-10-1 false positive fix: a local variable or function
+// parameter is scoped to the body of its enclosing function, not to the
+// namespace that function happens to be declared in. This matters for the
+// body of an explicit template specialization that C++ explicitly permits
+// users to add to namespace `std` (such as `std::hash<UserType>`): the
+// parameter and local names below are ordinary user-chosen identifiers and
+// do not themselves become new members of namespace `std`.
+struct UserTypeForHash {
+  int payload;
+};
+
+namespace std {
+template <typename T>
+struct hash; // forward declaration of the primary
+             // template that is specialized below
+
+template <> struct hash<UserTypeForHash> {
+  std::size_t
+  operator()(const UserTypeForHash &value) const noexcept { // COMPLIANT -
+                                                            // 'value' is a
+                                                            // parameter
+                                                            // scoped to the
+                                                            // function body,
+                                                            // not a member
+                                                            // of namespace
+                                                            // std
+    std::size_t result = static_cast<std::size_t>(
+        value.payload); // COMPLIANT - 'result' is a local variable scoped to
+                        // the function body, not a member of namespace std
+    return result;
+  }
+};
+
+// A genuinely new function declared directly in namespace std remains a
+// violation: this is not a permitted specialization, and it does introduce a
+// new name into the reserved namespace.
+void new_std_function() {} // NON_COMPLIANT - namespace std is reserved
+} // namespace std
\ No newline at end of file