diff --git a/icu4c/source/test/cintltst/ncnvtst.c b/icu4c/source/test/cintltst/ncnvtst.c
index 73251d153d9..41d3fa55b28 100644
--- a/icu4c/source/test/cintltst/ncnvtst.c
+++ b/icu4c/source/test/cintltst/ncnvtst.c
@@ -60,14 +60,14 @@ static void setNuConvTestName(const char *codepage, const char *direction)
 }
 
 
-static void TestSurrogateBehaviour();
-static void TestErrorBehaviour();
-static void TestToUnicodeErrorBehaviour();
-static void TestGetNextErrorBehaviour();
-static void TestRegressionUTF8();
-static void TestRegressionUTF32();
-static void TestAvailableConverters();
-static void TestFlushInternalBuffer();  /*for improved code coverage in ucnv_cnv.c*/
+static void TestSurrogateBehaviour(void);
+static void TestErrorBehaviour(void);
+static void TestToUnicodeErrorBehaviour(void);
+static void TestGetNextErrorBehaviour(void);
+static void TestRegressionUTF8(void);
+static void TestRegressionUTF32(void);
+static void TestAvailableConverters(void);
+static void TestFlushInternalBuffer(void);  /*for improved code coverage in ucnv_cnv.c*/
 
 static void TestWithBufferSize(int32_t osize, int32_t isize);
 
diff --git a/icu4c/source/test/cintltst/utf8tst.c b/icu4c/source/test/cintltst/utf8tst.c
index 4257ee36461..49bcdc810c8 100644
--- a/icu4c/source/test/cintltst/utf8tst.c
+++ b/icu4c/source/test/cintltst/utf8tst.c
@@ -22,13 +22,13 @@
 
 static void printUChars(uint8_t *uchars, int16_t len);
 
-static void TestCodeUnitValues();
-static void TestCharLength();
-static void TestGetChar();
-static void TestNextPrevChar();
-static void TestFwdBack();
-static void TestSetChar();
-static void TestAppendChar();
+static void TestCodeUnitValues(void);
+static void TestCharLength(void);
+static void TestGetChar(void);
+static void TestNextPrevChar(void);
+static void TestFwdBack(void);
+static void TestSetChar(void);
+static void TestAppendChar(void);
 
 
 
diff --git a/icu4c/source/test/cintltst/utransts.c b/icu4c/source/test/cintltst/utransts.c
index 336f993d9cc..ed8d31682d8 100644
--- a/icu4c/source/test/cintltst/utransts.c
+++ b/icu4c/source/test/cintltst/utransts.c
@@ -18,12 +18,12 @@
 
 #define TEST(x) addTest(root, &x, "utrans/" # x)
 
-static void TestAPI();
-static void TestSimpleRules();
-static void TestFilter();
-static void TestOpenInverse();
-static void TestClone();
-static void TestRegisterUnregister();
+static void TestAPI(void);
+static void TestSimpleRules(void);
+static void TestFilter(void);
+static void TestOpenInverse(void);
+static void TestClone(void);
+static void TestRegisterUnregister(void);
 
 static void _expectRules(const char*, const char*, const char*);
 static void _expect(const UTransliterator* trans, const char* cfrom, const char* cto);