diff --git a/src/node_buffer.cc b/src/node_buffer.cc
index 2778422ea4e7b7..2c6ddccc77dcfa 100644
--- a/src/node_buffer.cc
+++ b/src/node_buffer.cc
@@ -760,10 +760,23 @@ void StringWrite(const FunctionCallbackInfo<Value>& args) {
 
 void SlowByteLengthUtf8(const FunctionCallbackInfo<Value>& args) {
   CHECK(args[0]->IsString());
-
-  // Fast case: avoid StringBytes on UTF8 string. Jump to v8.
-  size_t result = args[0].As<String>()->Utf8LengthV2(args.GetIsolate());
-  args.GetReturnValue().Set(static_cast<uint64_t>(result));
+  Isolate* isolate = args.GetIsolate();
+  Local<String> str = args[0].As<String>();
+
+  // Below ~512 units, or for one-byte, V8's Utf8LengthV2 is faster.
+  if (str->Length() >= 512 && !str->IsOneByte()) {
+    String::ValueView view(isolate, str);
+    if (!view.is_one_byte()) {
+      // with_replacement matches Buffer.from's U+FFFD for lone surrogates.
+      size_t result =
+          simdutf::utf8_length_from_utf16_with_replacement(
+              reinterpret_cast<const char16_t*>(view.data16()), view.length())
+              .count;
+      args.GetReturnValue().Set(static_cast<uint64_t>(result));
+      return;
+    }
+  }
+  args.GetReturnValue().Set(static_cast<uint64_t>(str->Utf8LengthV2(isolate)));
 }
 
 uint32_t FastByteLengthUtf8(
@@ -777,6 +790,17 @@ uint32_t FastByteLengthUtf8(
   Local<String> sourceStr = sourceValue.As<String>();
 
   if (!sourceStr->IsExternalOneByte()) {
+    // Below ~512 units, or for one-byte, V8's Utf8LengthV2 is faster.
+    if (sourceStr->Length() >= 512 && !sourceStr->IsOneByte()) {
+      String::ValueView view(isolate, sourceStr);
+      if (!view.is_one_byte()) {
+        // with_replacement matches Buffer.from's U+FFFD for lone surrogates.
+        return simdutf::utf8_length_from_utf16_with_replacement(
+                   reinterpret_cast<const char16_t*>(view.data16()),
+                   view.length())
+            .count;
+      }
+    }
     return sourceStr->Utf8LengthV2(isolate);
   }
   auto source = sourceStr->GetExternalOneByteStringResource();
diff --git a/test/parallel/test-buffer-bytelength.js b/test/parallel/test-buffer-bytelength.js
index fec508ffb4048a..267c1e26e876ab 100644
--- a/test/parallel/test-buffer-bytelength.js
+++ b/test/parallel/test-buffer-bytelength.js
@@ -125,3 +125,49 @@ for (let i = 1; i < 10; i++) {
   assert.strictEqual(Buffer.byteLength('foo', encoding),
                      Buffer.byteLength('foo', 'utf8'));
 }
+
+// byteLength('utf8') must equal the bytes Buffer.from writes, including
+// unpaired surrogates (3-byte U+FFFD). Large inputs exercise the SIMD path.
+{
+  const HI = '\uD83D'; // Unpaired high surrogate
+  const LO = '\uDE00'; // Unpaired low surrogate
+  const PAIR = '\u{1F600}'; // Valid surrogate pair (4 bytes)
+  const enc = new TextEncoder();
+
+  // Independent UTF-8 byte-length oracle with replacement semantics.
+  const utf8Bytes = (str) => {
+    let n = 0;
+    for (let i = 0; i < str.length; i++) {
+      const c = str.charCodeAt(i);
+      if (c < 0x80) n += 1;
+      else if (c < 0x800) n += 2;
+      else if (c >= 0xD800 && c <= 0xDBFF) {
+        const next = str.charCodeAt(i + 1);
+        if (next >= 0xDC00 && next <= 0xDFFF) { n += 4; i++; } else n += 3;
+      } else n += 3; // BMP >= 0x800, incl. unpaired low surrogate
+    }
+    return n;
+  };
+
+  const cases = [
+    'a'.repeat(300) + HI + 'b'.repeat(300), // Unpaired high inside large 2-byte
+    'a'.repeat(300) + LO + 'b'.repeat(300), // Unpaired low inside large 2-byte
+    HI.repeat(200), // many unpaired highs
+    (HI + LO).repeat(200), // Reversed order, still unpaired
+    PAIR.repeat(200), // valid pairs, large
+    '中'.repeat(500), // BMP 3-byte, large
+    'é'.repeat(500), // latin1, large
+    `a中${PAIR}${HI}é`.repeat(100), // mixed, large
+    `A${HI}`, // Tiny, below SIMD threshold
+    HI, // Single unpaired surrogate
+    '',
+  ];
+
+  for (const s of cases) {
+    const ref = enc.encode(s); // WHATWG ground-truth UTF-8 bytes
+    const label = JSON.stringify(s).slice(0, 32);
+    assert.strictEqual(utf8Bytes(s), ref.length, `oracle vs TextEncoder: ${label}`);
+    assert.strictEqual(Buffer.byteLength(s, 'utf8'), ref.length, `byteLength: ${label}`);
+    assert.deepStrictEqual(new Uint8Array(Buffer.from(s, 'utf8')), ref, `bytes: ${label}`);
+  }
+}