Index: include/clang/Driver/CC1Options.td
===================================================================
--- include/clang/Driver/CC1Options.td
+++ include/clang/Driver/CC1Options.td
@@ -437,6 +437,8 @@
   HelpText<"Include code patterns in code-completion results">;
 def no_code_completion_globals : Flag<["-"], "no-code-completion-globals">,
   HelpText<"Do not include global declarations in code-completion results.">;
+def no_code_completion_ns_level_decls : Flag<["-"], "no-code-completion-ns-level-decls">,
+  HelpText<"Do not include declarations inside namespaces (incl. global namespace) in the code-completion results.">;
 def code_completion_brief_comments : Flag<["-"], "code-completion-brief-comments">,
   HelpText<"Include brief documentation comments in code-completion results.">;
 def disable_free : Flag<["-"], "disable-free">,
Index: include/clang/Sema/CodeCompleteConsumer.h
===================================================================
--- include/clang/Sema/CodeCompleteConsumer.h
+++ include/clang/Sema/CodeCompleteConsumer.h
@@ -919,8 +919,13 @@
   }
 
   /// \brief Whether to include global (top-level) declaration results.
-  bool includeGlobals() const {
-    return CodeCompleteOpts.IncludeGlobals;
+  bool includeGlobals() const { return CodeCompleteOpts.IncludeGlobals; }
+
+  /// \brief Whether to include declarations in namespace contexts (including
+  /// the global namespace). If this is false, `includeGlobals()` will be
+  /// ignored.
+  bool includeNamespaceLevelDecls() const {
+    return CodeCompleteOpts.IncludeNamespaceLevelDecls;
   }
 
   /// \brief Whether to include brief documentation comments within the set of
Index: include/clang/Sema/CodeCompleteOptions.h
===================================================================
--- include/clang/Sema/CodeCompleteOptions.h
+++ include/clang/Sema/CodeCompleteOptions.h
@@ -24,15 +24,20 @@
   /// Show top-level decls in code completion results.
   unsigned IncludeGlobals : 1;
 
+  /// Show decls in namespace (including the global namespace) in code
+  /// completion results. If this is 0, `IncludeGlobals` will be ignored.
+  ///
+  /// Currently, this only works when completing qualified IDs (i.e.
+  /// `Sema::CodeCompleteQualifiedId`).
+  /// FIXME: consider supporting more completion cases with this option.
+  unsigned IncludeNamespaceLevelDecls : 1;
+
   /// Show brief documentation comments in code completion results.
   unsigned IncludeBriefComments : 1;
 
-  CodeCompleteOptions() :
-      IncludeMacros(0),
-      IncludeCodePatterns(0),
-      IncludeGlobals(1),
-      IncludeBriefComments(0)
-  { }
+  CodeCompleteOptions()
+      : IncludeMacros(0), IncludeCodePatterns(0), IncludeGlobals(1),
+        IncludeNamespaceLevelDecls(1), IncludeBriefComments(0) {}
 };
 
 } // namespace clang
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -1380,6 +1380,8 @@
     = Args.hasArg(OPT_code_completion_patterns);
   Opts.CodeCompleteOpts.IncludeGlobals
     = !Args.hasArg(OPT_no_code_completion_globals);
+  Opts.CodeCompleteOpts.IncludeNamespaceLevelDecls
+    = !Args.hasArg(OPT_no_code_completion_ns_level_decls);
   Opts.CodeCompleteOpts.IncludeBriefComments
     = Args.hasArg(OPT_code_completion_brief_comments);
 
Index: lib/Sema/SemaCodeComplete.cpp
===================================================================
--- lib/Sema/SemaCodeComplete.cpp
+++ lib/Sema/SemaCodeComplete.cpp
@@ -4647,10 +4647,13 @@
     MaybeAddOverrideCalls(*this, Ctx, Results);
   Results.ExitScope();
 
-  CodeCompletionDeclConsumer Consumer(Results, CurContext);
-  LookupVisibleDecls(Ctx, LookupOrdinaryName, Consumer,
-                     /*IncludeGlobalScope=*/true,
-                     /*IncludeDependentBases=*/true);
+  if (CodeCompleter->includeNamespaceLevelDecls() ||
+      (!Ctx->isNamespace() && !Ctx->isTranslationUnit())) {
+    CodeCompletionDeclConsumer Consumer(Results, CurContext);
+    LookupVisibleDecls(Ctx, LookupOrdinaryName, Consumer,
+                       /*IncludeGlobalScope=*/true,
+                       /*IncludeDependentBases=*/true);
+  }
 
   auto CC = Results.getCompletionContext();
   CC.setCXXScopeSpecifier(SS);
Index: test/CodeCompletion/ignore-ns-level-decls.cpp
===================================================================
--- /dev/null
+++ test/CodeCompletion/ignore-ns-level-decls.cpp
@@ -0,0 +1,21 @@
+namespace ns {
+  struct bar {
+  };
+
+  struct baz {
+  };
+
+  int func(int a, bar b, baz c);
+}
+
+void test() {
+  ns::
+// RUN: %clang_cc1 -fsyntax-only -code-completion-at=%s:12:7 %s -o - | FileCheck %s --check-prefix=CHECK-1
+// CHECK-1-DAG: COMPLETION: bar : bar
+// CHECK-1-DAG: COMPLETION: baz : baz
+// CHECK-1-DAG: COMPLETION: func : [#int#]func(<#int a#>, <#bar b#>, <#baz c#>)
+
+// RUN: %clang_cc1 -fsyntax-only -code-completion-at=%s:12:7 -no-code-completion-ns-level-decls %s -o - | FileCheck %s --check-prefix=CHECK-EMPTY
+// CHECK-EMPTY-NOT: COMPLETION: bar : bar
+// CHECK-EMPTY: {{^}}{{$}}
+}