diff --git a/arch/riscv/lib/strnlen.S b/arch/riscv/lib/strnlen.S
index 53afa7b5b314..fc5d578bbffe 100644
--- a/arch/riscv/lib/strnlen.S
+++ b/arch/riscv/lib/strnlen.S
@@ -17,6 +17,7 @@ SYM_FUNC_START(strnlen)
 	__ALTERNATIVE_CFG("nop", "j strnlen_zbb", 0, RISCV_ISA_EXT_ZBB,
 		IS_ENABLED(CONFIG_RISCV_ISA_ZBB) && IS_ENABLED(CONFIG_TOOLCHAIN_HAS_ZBB))
 
+strnlen_generic:
 
 	/*
 	 * Returns
@@ -27,20 +28,24 @@ SYM_FUNC_START(strnlen)
 	 *   a1 - Max length of string
 	 *
 	 * Clobbers
-	 *   t0, t1, t2
+	 *   t0, t1
 	 */
-	addi	t1, a0, -1
-	add	t2, a0, a1
+	mv	t1, a0
+
 1:
-	addi	t1, t1, 1
-	beq	t1, t2, 2f
+	beqz	a1, 2f
+	addi	a1, a1, -1
+
 	lbu	t0, 0(t1)
-	bnez	t0, 1b
+	beqz	t0, 2f
+
+	addi	t1, t1, 1
+	j	1b
+
 2:
 	sub	a0, t1, a0
 	ret
 
-
 /*
  * Variant of strnlen using the ZBB extension if available
  */
@@ -73,6 +78,16 @@ strnlen_zbb:
 	/* If maxlen is 0, return 0. */
 	beqz	a1, 3f
 
+	/*
+	 * Fallback to generic implementation when count is large enough to
+	 * cause address overflow in the ZBB optimized path, or when count
+	 * equals SIZE_MAX where the minu instruction misbehaves.
+	 */
+	li	t5, -1
+	beq	a1, t5, strnlen_generic		/* count == SIZE_MAX */
+	add	t4, a0, a1
+	bltu	t4, a0, strnlen_generic		/* a0 + a1 overflow */
+
 	/* Number of irrelevant bytes in the first word. */
 	andi	t2, a0, SZREG-1
 
@@ -83,8 +98,12 @@ strnlen_zbb:
 	sub	t3, t3, t2
 	slli	t2, t2, 3
 
-	/* Aligned boundary. */
-	add	t4, a0, a1
+	/*
+	 * Aligned boundary.  Use the address of the last valid byte
+	 * (s + count - 1) to avoid loading a word past the count
+	 * boundary in the loop below.  count == 0 is handled above.
+	 */
+	addi	t4, t4, -1
 	andi	t4, t4, -SZREG
 
 	/* Get the first word.  */
@@ -120,6 +139,9 @@ strnlen_zbb:
 
 	bgtu	t3, a0, 2f
 
+	/* All remaining bytes are in the first word, no loop needed. */
+	bgeu	t0, t4, 2f
+
 	/* Prepare for the word comparison loop. */
 	addi	t2, t0, SZREG
 	li	t3, -1
diff --git a/lib/string_kunit.c b/lib/string_kunit.c
index 2a2e95886ad1..d00932ee457a 100644
--- a/lib/string_kunit.c
+++ b/lib/string_kunit.c
@@ -155,6 +155,16 @@ static void string_test_strnlen(struct kunit *test)
 
 	for (size_t offset = 0; offset < STRING_TEST_MAX_OFFSET; offset++) {
 		for (size_t len = 0; len <= STRING_TEST_MAX_LEN; len++) {
+			/* Test strings without NUL terminator */
+			s = buf + buf_size - offset - len;
+			if (len > 0)
+				KUNIT_EXPECT_EQ(test, strnlen(s, len - 1), len - 1);
+			if (len > 1)
+				KUNIT_EXPECT_EQ(test, strnlen(s, len - 2), len - 2);
+
+			KUNIT_EXPECT_EQ(test, strnlen(s, len), len);
+
+			/* Test strings with NUL terminator */
 			s = buf + buf_size - 1 - offset - len;
 			s[len] = '\0';
 
@@ -169,6 +179,9 @@ static void string_test_strnlen(struct kunit *test)
 			KUNIT_EXPECT_EQ(test, strnlen(s, len + 2), len);
 			KUNIT_EXPECT_EQ(test, strnlen(s, len + 10), len);
 
+			/* Test Count overflow fallback */
+			KUNIT_EXPECT_EQ(test, strnlen(s, SSIZE_MAX), len);
+
 			s[len] = 'A';
 		}
 	}