Index: clang-tidy/misc/StaticAssertCheck.cpp
===================================================================
--- clang-tidy/misc/StaticAssertCheck.cpp
+++ clang-tidy/misc/StaticAssertCheck.cpp
@@ -27,10 +27,9 @@
     : ClangTidyCheck(Name, Context) {}
 
 void StaticAssertCheck::registerMatchers(MatchFinder *Finder) {
-  // FIXME: I don't see why this checker couldn't also be interesting for
-  // _Static_assert in C11, or static_assert if <assert.h> has been included, 
-  // but it is currently only enabled for C++11. Investigate.
-  if (getLangOpts().CPlusPlus11) {
+  // This checker only makes sense for languages that have static assertion
+  // capabilities: C++11 and C11.
+  if (getLangOpts().CPlusPlus11 || getLangOpts().C11) {
     auto IsAlwaysFalse = expr(ignoringParenImpCasts(
         expr(anyOf(boolLiteral(equals(false)), integerLiteral(equals(0)),
                    nullPtrLiteralExpr(), gnuNullExpr()))
@@ -108,6 +107,14 @@
       return;
   }
 
+  // FIXME: static_assert is fine in C11 if the macro is defined, otherwise we
+  // should suggest _Static_assert. However, there is no way to determine
+  // whether the macro is defined at the location of the assert without access
+  // to the Preprocessor instance. For right now, always suggest _Static_assert
+  // in C11 mode.
+  StringRef Replacement =
+      getLangOpts().C11 ? "_Static_assert" : "static_assert";
+
   SourceLocation AssertLoc = SM.getImmediateMacroCallerLoc(AssertExpansionLoc);
 
   SmallVector<FixItHint, 4> FixItHints;
@@ -115,7 +122,7 @@
   if (AssertLoc.isValid() && !AssertLoc.isMacroID() &&
       (LastParenLoc = getLastParenLoc(ASTCtx, AssertLoc)).isValid()) {
     FixItHints.push_back(
-        FixItHint::CreateReplacement(SourceRange(AssertLoc), "static_assert"));
+        FixItHint::CreateReplacement(SourceRange(AssertLoc), Replacement));
 
     std::string StaticAssertMSG = ", \"\"";
     if (AssertExprRoot) {
@@ -130,8 +137,8 @@
         FixItHint::CreateInsertion(LastParenLoc, StaticAssertMSG));
   }
 
-  diag(AssertLoc, "found assert() that could be replaced by static_assert()")
-      << FixItHints;
+  diag(AssertLoc, "found assert() that could be replaced by %0()")
+      << Replacement << FixItHints;
 }
 
 SourceLocation StaticAssertCheck::getLastParenLoc(const ASTContext *ASTCtx,
Index: test/clang-tidy/check_clang_tidy.py
===================================================================
--- test/clang-tidy/check_clang_tidy.py
+++ test/clang-tidy/check_clang_tidy.py
@@ -38,12 +38,16 @@
     sys.exit('Not enough arguments.')
 
   input_file_name = sys.argv[1]
+  extension = '.cpp'
+  if (input_file_name.endswith('.c')):
+    extension = '.c'
+    
   check_name = sys.argv[2]
-  temp_file_name = sys.argv[3] + '.cpp'
+  temp_file_name = sys.argv[3] + extension
 
   clang_tidy_extra_args = sys.argv[4:]
   if len(clang_tidy_extra_args) == 0:
-    clang_tidy_extra_args = ['--', '--std=c++11']
+    clang_tidy_extra_args = ['--', '--std=c++11' if extension == '.cpp' else '-std=c99']
 
   with open(input_file_name, 'r') as input_file:
     input_text = input_file.read()
Index: test/clang-tidy/misc-static-assert-c11.c
===================================================================
--- test/clang-tidy/misc-static-assert-c11.c
+++ test/clang-tidy/misc-static-assert-c11.c
@@ -0,0 +1,26 @@
+// RUN: %python %S/check_clang_tidy.py %s misc-static-assert %t -- -std=c11
+
+void abort() {}
+#ifdef NDEBUG
+#define assert(x) 1
+#else
+#define assert(x)                                                              \
+  if (!(x))                                                                    \
+  abort()
+#endif
+
+void f(void) {
+  int x = 1;
+  assert(x == 0);
+  // CHECK-FIXES: {{^  }}assert(x == 0);
+
+  #define static_assert(x, msg) _Static_assert(x, msg)
+  assert(11 == 5 + 6);
+  // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: found assert() that could be
+  // CHECK-FIXES: {{^  }}_Static_assert(11 == 5 + 6, "");
+  #undef static_assert
+
+  assert(10 == 5 + 5);
+  // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: found assert() that could be
+  // CHECK-FIXES: {{^  }}_Static_assert(10 == 5 + 5, "");
+}
Index: test/clang-tidy/misc-static-assert-c99.c
===================================================================
--- test/clang-tidy/misc-static-assert-c99.c
+++ test/clang-tidy/misc-static-assert-c99.c
@@ -0,0 +1,24 @@
+// RUN: %python %S/check_clang_tidy.py %s misc-static-assert %t -- -std=c99
+
+void abort() {}
+#ifdef NDEBUG
+#define assert(x) 1
+#else
+#define assert(x)                                                              \
+  if (!(x))                                                                    \
+  abort()
+#endif
+
+void f(void) {
+  int x = 1;
+  assert(x == 0);
+  // CHECK-FIXES: {{^  }}assert(x == 0);
+
+  #define static_assert(x, msg) _Static_assert(x, msg)
+  assert(11 == 5 + 6);
+  // CHECK-FIXES: {{^  }}assert(11 == 5 + 6);
+  #undef static_assert
+
+  assert(10 == 5 + 5);
+  // CHECK-FIXES: {{^  }}assert(10 == 5 + 5);
+}