diff --git a/clang-tools-extra/clang-tidy/utils/RenamerClangTidyCheck.cpp b/clang-tools-extra/clang-tidy/utils/RenamerClangTidyCheck.cpp
--- a/clang-tools-extra/clang-tidy/utils/RenamerClangTidyCheck.cpp
+++ b/clang-tools-extra/clang-tidy/utils/RenamerClangTidyCheck.cpp
@@ -272,6 +272,11 @@
   }
 
   if (const auto *Decl = Result.Nodes.getNodeAs<NamedDecl>("decl")) {
+    // Fix using namespace declarations.
+    if (const auto *UsingNS = dyn_cast<UsingDirectiveDecl>(Decl))
+      addUsage(NamingCheckFailures, UsingNS->getNominatedNamespaceAsWritten(),
+               UsingNS->getIdentLocation());
+
     if (!Decl->getIdentifier() || Decl->getName().empty() || Decl->isImplicit())
       return;
 
diff --git a/clang-tools-extra/test/clang-tidy/checkers/readability-identifier-naming.cpp b/clang-tools-extra/test/clang-tidy/checkers/readability-identifier-naming.cpp
--- a/clang-tools-extra/test/clang-tidy/checkers/readability-identifier-naming.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/readability-identifier-naming.cpp
@@ -527,3 +527,8 @@
 // CHECK-FIXES: {{^}}    int * const lc_PointerB = nullptr;{{$}}
 }
 
+using namespace FOO_NS;
+// CHECK-FIXES: {{^}}using namespace foo_ns;
+
+using namespace FOO_NS::InlineNamespace;
+// CHECK-FIXES: {{^}}using namespace foo_ns::inline_namespace;