diff --git a/src/node_buffer.cc b/src/node_buffer.cc
index d4a63cf610ca7f..79fd433b712355 100644
--- a/src/node_buffer.cc
+++ b/src/node_buffer.cc
@@ -587,9 +587,9 @@ void StringSlice(const FunctionCallbackInfo<Value>& args) {
 
 void CopyImpl(Local<Value> source_obj,
               Local<Value> target_obj,
-              const uint32_t target_start,
-              const uint32_t source_start,
-              const uint32_t to_copy) {
+              const size_t target_start,
+              const size_t source_start,
+              const size_t to_copy) {
   ArrayBufferViewContents<char> source(source_obj);
   SPREAD_BUFFER_ARG(target_obj, target);
 
@@ -598,24 +598,32 @@ void CopyImpl(Local<Value> source_obj,
 
 // Assume caller has properly validated args.
 void SlowCopy(const FunctionCallbackInfo<Value>& args) {
+  Environment* env = Environment::GetCurrent(args);
   Local<Value> source_obj = args[0];
   Local<Value> target_obj = args[1];
-  const uint32_t target_start = args[2].As<Uint32>()->Value();
-  const uint32_t source_start = args[3].As<Uint32>()->Value();
-  const uint32_t to_copy = args[4].As<Uint32>()->Value();
+  int64_t target_start, source_start, to_copy;
+  if (!args[2]->IntegerValue(env->context()).To(&target_start) ||
+      !args[3]->IntegerValue(env->context()).To(&source_start) ||
+      !args[4]->IntegerValue(env->context()).To(&to_copy)) {
+    return;
+  }
 
-  CopyImpl(source_obj, target_obj, target_start, source_start, to_copy);
+  CopyImpl(source_obj,
+           target_obj,
+           static_cast<size_t>(target_start),
+           static_cast<size_t>(source_start),
+           static_cast<size_t>(to_copy));
 
-  args.GetReturnValue().Set(to_copy);
+  args.GetReturnValue().Set(static_cast<double>(to_copy));
 }
 
 // Assume caller has properly validated args.
-uint32_t FastCopy(Local<Value> receiver,
+uint64_t FastCopy(Local<Value> receiver,
                   Local<Value> source_obj,
                   Local<Value> target_obj,
-                  uint32_t target_start,
-                  uint32_t source_start,
-                  uint32_t to_copy,
+                  uint64_t target_start,
+                  uint64_t source_start,
+                  uint64_t to_copy,
                   // NOLINTNEXTLINE(runtime/references)
                   FastApiCallbackOptions& options) {
   HandleScope scope(options.isolate);
diff --git a/test/parallel/test-buffer-copy-large.js b/test/parallel/test-buffer-copy-large.js
new file mode 100644
index 00000000000000..5580fc455fcbda
--- /dev/null
+++ b/test/parallel/test-buffer-copy-large.js
@@ -0,0 +1,44 @@
+'use strict';
+
+// Regression test for https://github.com/nodejs/node/issues/55422
+// Buffer.copy and Buffer.concat silently produced incorrect results when
+// indices involved were >= 2^32 due to 32-bit integer overflow in SlowCopy.
+
+const common = require('../common');
+const assert = require('assert');
+
+// This test exercises the native SlowCopy path in node_buffer.cc.
+// SlowCopy is invoked by _copyActual when the fast TypedArrayPrototypeSet
+// path cannot be used (i.e. when sourceStart !== 0 or nb !== sourceLen).
+
+// Cannot test on 32-bit machines since buffers that large cannot exist there.
+common.skipIf32Bits();
+
+const THRESHOLD = 2 ** 32; // 4 GiB
+
+// Allocate a large target buffer (just over 4 GiB) to test that a targetStart
+// value >= 2^32 is not silently truncated to its lower 32 bits.
+let target;
+try {
+  target = Buffer.alloc(THRESHOLD + 10, 0);
+} catch (e) {
+  if (e.code === 'ERR_MEMORY_ALLOCATION_FAILED' ||
+      /Array buffer allocation failed/.test(e.message)) {
+    common.skip('insufficient memory for large buffer allocation');
+  }
+  throw e;
+}
+
+const source = Buffer.alloc(10, 111);
+
+// Copy only the first 5 bytes of source (nb=5, sourceLen=10 → nb !== sourceLen)
+// so _copyActual falls through to the native _copy (SlowCopy) instead of
+// using TypedArrayPrototypeSet.  The targetStart is THRESHOLD (2^32), which
+// previously overflowed to 0 when cast to uint32_t.
+source.copy(target, THRESHOLD, 0, 5);
+
+// The 5 copied bytes must appear at position THRESHOLD, not at position 0.
+assert.strictEqual(target[0], 0, 'position 0 must not have been overwritten');
+assert.strictEqual(target[THRESHOLD], 111, 'byte at THRESHOLD must be 111');
+assert.strictEqual(target[THRESHOLD + 4], 111, 'byte at THRESHOLD+4 must be 111');
+assert.strictEqual(target[THRESHOLD + 5], 0, 'byte at THRESHOLD+5 must be 0 (not copied)');