diff --git a/tests/suites/test_suite_chacha20.data b/tests/suites/test_suite_chacha20.data
index 86094604bf..d28d771264 100644
--- a/tests/suites/test_suite_chacha20.data
+++ b/tests/suites/test_suite_chacha20.data
@@ -22,5 +22,8 @@ chacha20_crypt:"1c9240a5eb55d38af333888604f6b5f0473917c1402b80099dca5cbc207075c0
 ChaCha20 RFC 7539 Test Vector #3 (Decrypt)
 chacha20_crypt:"1c9240a5eb55d38af333888604f6b5f0473917c1402b80099dca5cbc207075c0":"000000000000000000000002":42:"62e6347f95ed87a45ffae7426f27a1df5fb69110044c0d73118effa95b01e5cf166d3df2d721caf9b21e5fb14c616871fd84c54f9d65b283196c7fe4f60553ebf39c6402c42234e32a356b3e764312a61a5532055716ead6962568f87d3f3f7704c6a8d1bcd1bf4d50d6154b6da731b187b58dfd728afa36757a797ac188d1":"2754776173206272696c6c69672c20616e642074686520736c6974687920746f7665730a446964206779726520616e642067696d626c6520696e2074686520776162653a0a416c6c206d696d737920776572652074686520626f726f676f7665732c0a416e6420746865206d6f6d65207261746873206f757467726162652e"
 
+ChaCha20 Counter Overflow
+chacha20_input_len_too_long:
+
 ChaCha20 Selftest
 chacha20_self_test:
diff --git a/tests/suites/test_suite_chacha20.function b/tests/suites/test_suite_chacha20.function
index d6b67e12f2..0e8dd91324 100644
--- a/tests/suites/test_suite_chacha20.function
+++ b/tests/suites/test_suite_chacha20.function
@@ -67,6 +67,47 @@ void chacha20_crypt(data_t *key_str,
 }
 /* END_CASE */
 
+/* BEGIN_CASE */
+void chacha20_input_len_too_long(void)
+{
+    mbedtls_chacha20_context ctx;
+    unsigned char key[32] = { 0 };
+    unsigned char nonce[12] = { 0 };
+    unsigned char input[65] = { 0 };
+    unsigned char output[65] = { 0 };
+    size_t i;
+
+    mbedtls_chacha20_init(&ctx);
+
+    TEST_ASSERT(mbedtls_chacha20_setkey(&ctx, key) == 0);
+    TEST_ASSERT(mbedtls_chacha20_starts(&ctx, nonce, UINT32_MAX) == 0);
+
+    /*
+     * An overlong call must fail before consuming buffered keystream bytes,
+     * writing partial output, or advancing the context.
+     */
+    TEST_EQUAL(mbedtls_chacha20_update(&ctx, 1, input, output), 0);
+    memset(output, 0x2a, sizeof(output));
+    TEST_EQUAL(mbedtls_chacha20_update(&ctx, 64, input, output),
+               MBEDTLS_ERR_CHACHA20_BAD_INPUT_DATA);
+    for (i = 0; i < sizeof(output); i++) {
+        TEST_EQUAL(output[i], 0x2a);
+    }
+
+    /* Consume the rest of the final counter block, then reject more input. */
+    TEST_EQUAL(mbedtls_chacha20_update(&ctx, 63, input, output), 0);
+    TEST_EQUAL(mbedtls_chacha20_update(&ctx, 1, input, output),
+               MBEDTLS_ERR_CHACHA20_BAD_INPUT_DATA);
+
+    /* The one-shot API should report the same error. */
+    TEST_EQUAL(mbedtls_chacha20_crypt(key, nonce, UINT32_MAX,
+                                      sizeof(input), input, output),
+               MBEDTLS_ERR_CHACHA20_BAD_INPUT_DATA);
+
+    mbedtls_chacha20_free(&ctx);
+}
+/* END_CASE */
+
 /* BEGIN_CASE depends_on:MBEDTLS_SELF_TEST */
 void chacha20_self_test()
 {