diff --git a/guava-tests/test/com/google/common/math/IntMathTest.java b/guava-tests/test/com/google/common/math/IntMathTest.java
index fa8b3d4c4ab5..126a32f4afff 100644
--- a/guava-tests/test/com/google/common/math/IntMathTest.java
+++ b/guava-tests/test/com/google/common/math/IntMathTest.java
@@ -25,6 +25,7 @@
 import static com.google.common.math.MathTesting.POSITIVE_INTEGER_CANDIDATES;
 import static com.google.common.math.ReflectionFreeAssertThrows.assertThrows;
 import static com.google.common.math.TestPlatform.intsCanGoOutOfRange;
+import static com.google.common.truth.Truth.assertWithMessage;
 import static java.lang.Math.min;
 import static java.math.BigInteger.valueOf;
 import static java.math.RoundingMode.FLOOR;
@@ -373,12 +374,14 @@ public void testDivByZeroAlwaysFails() {
   public void testMod() {
     for (int x : ALL_INTEGER_CANDIDATES) {
       for (int m : POSITIVE_INTEGER_CANDIDATES) {
-        assertEquals(valueOf(x).mod(valueOf(m)).intValue(), IntMath.mod(x, m));
+        assertWithMessage("%s mod %s", x, m)
+            .that(IntMath.mod(x, m))
+            .isEqualTo(valueOf(x).mod(valueOf(m)).intValue());
       }
     }
   }
 
-  public void testModNegativeModulusFails() {
+  public void testModNegativeModulus() {
     for (int x : POSITIVE_INTEGER_CANDIDATES) {
       for (int m : NEGATIVE_INTEGER_CANDIDATES) {
         assertThrows(ArithmeticException.class, () -> IntMath.mod(x, m));
diff --git a/guava-tests/test/com/google/common/math/LongMathTest.java b/guava-tests/test/com/google/common/math/LongMathTest.java
index f88baa99f727..7aae34ed1b2a 100644
--- a/guava-tests/test/com/google/common/math/LongMathTest.java
+++ b/guava-tests/test/com/google/common/math/LongMathTest.java
@@ -448,7 +448,7 @@ public void testMod() {
   }
 
   @GwtIncompatible // TODO
-  public void testModNegativeModulusFails() {
+  public void testModNegativeModulus() {
     for (long x : ALL_LONG_CANDIDATES) {
       for (long m : NEGATIVE_LONG_CANDIDATES) {
         assertThrows(ArithmeticException.class, () -> LongMath.mod(x, m));
diff --git a/guava/src/com/google/common/math/IntMath.java b/guava/src/com/google/common/math/IntMath.java
index fa0d782ea4bf..aacc89e0a5a3 100644
--- a/guava/src/com/google/common/math/IntMath.java
+++ b/guava/src/com/google/common/math/IntMath.java
@@ -385,8 +385,7 @@ public static int mod(int x, int m) {
     if (m <= 0) {
       throw new ArithmeticException("Modulus " + m + " must be > 0");
     }
-    int result = x % m;
-    return (result >= 0) ? result : result + m;
+    return Math.floorMod(x, m);
   }
 
   /**
diff --git a/guava/src/com/google/common/math/LongMath.java b/guava/src/com/google/common/math/LongMath.java
index 65eed0a98cd0..51feebf95cc7 100644
--- a/guava/src/com/google/common/math/LongMath.java
+++ b/guava/src/com/google/common/math/LongMath.java
@@ -473,8 +473,7 @@ public static long mod(long x, long m) {
     if (m <= 0) {
       throw new ArithmeticException("Modulus must be positive");
     }
-    long result = x % m;
-    return (result >= 0) ? result : result + m;
+    return Math.floorMod(x, m);
   }
 
   /**