diff --git a/src/runtime/node/zlib/NativeZstd.rs b/src/runtime/node/zlib/NativeZstd.rs
index 287f782dcc37..a8386524747e 100644
--- a/src/runtime/node/zlib/NativeZstd.rs
+++ b/src/runtime/node/zlib/NativeZstd.rs
@@ -197,14 +197,18 @@ mod _impl {
                 write_js_callback.with_async_context_if_needed(global),
             );
 
+            // `u64::MAX` is `ZSTD_CONTENTSIZE_UNKNOWN`. `ZSTD_CCtx_setPledgedSrcSize`
+            // takes a u64, so the only bound is what a JS number represents exactly.
             let mut pledged_src_size: u64 = u64::MAX;
             if pledged_src_size_value.is_number() {
-                pledged_src_size = u64::from(validators::validate_uint32(
+                pledged_src_size = u64::try_from(validators::validate_integer(
                     global,
                     pledged_src_size_value,
-                    format_args!("pledgedSrcSize"),
-                    false,
-                )?);
+                    "pledgedSrcSize",
+                    Some(0),
+                    Some(jsc::MAX_SAFE_INTEGER),
+                )?)
+                .expect("infallible: validated range");
             }
 
             let err = self.stream.with_mut(|s| s.init(pledged_src_size));
diff --git a/test/js/node/zlib/zstd-pledged-src-size.test.ts b/test/js/node/zlib/zstd-pledged-src-size.test.ts
new file mode 100644
index 000000000000..32cb04c4fd13
--- /dev/null
+++ b/test/js/node/zlib/zstd-pledged-src-size.test.ts
@@ -0,0 +1,72 @@
+import { describe, expect, it } from "bun:test";
+import { promisify } from "node:util";
+import * as zlib from "node:zlib";
+
+// https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#frame_header
+// magic(4) | frame header descriptor(1) | [window descriptor(1)] | [dictionary id] | [frame content size]
+function frameContentSize(frame: Buffer): bigint | null {
+  expect(frame.readUInt32LE(0)).toBe(0xfd2fb528);
+  const descriptor = frame[4];
+  const singleSegment = (descriptor >> 5) & 1;
+  const offset = 5 + (singleSegment ? 0 : 1) + [0, 1, 2, 4][descriptor & 0b11];
+  switch (descriptor >> 6) {
+    case 0:
+      return singleSegment ? BigInt(frame[offset]) : null;
+    case 1:
+      return BigInt(frame.readUInt16LE(offset) + 256);
+    case 2:
+      return BigInt(frame.readUInt32LE(offset));
+    default:
+      return frame.readBigUInt64LE(offset);
+  }
+}
+
+// Finishing with ZSTD_e_flush instead of ZSTD_e_end means zstd never checks the pledged size
+// against what was actually written, so these can pledge 4 GiB without feeding it 4 GiB.
+// The pledge still goes into the frame header.
+const input = Buffer.alloc(64, 0x61);
+const flushOnly = (pledgedSrcSize: number) => ({ pledgedSrcSize, finishFlush: zlib.constants.ZSTD_e_flush });
+
+describe("zstd pledgedSrcSize", () => {
+  it("is written to the frame header unmodified when below 4 GiB", () => {
+    expect(frameContentSize(zlib.zstdCompressSync(input, flushOnly(1000)))).toBe(1000n);
+    expect(frameContentSize(zlib.zstdCompressSync(input, flushOnly(2 ** 32 - 1)))).toBe(4294967295n);
+  });
+
+  it("accepts 4 GiB and larger", () => {
+    expect(frameContentSize(zlib.zstdCompressSync(input, flushOnly(2 ** 32)))).toBe(4294967296n);
+    expect(frameContentSize(zlib.zstdCompressSync(input, flushOnly(2 ** 33 + 1)))).toBe(8589934593n);
+    expect(frameContentSize(zlib.zstdCompressSync(input, flushOnly(Number.MAX_SAFE_INTEGER)))).toBe(9007199254740991n);
+  });
+
+  it("accepts 4 GiB and larger when compressing asynchronously", async () => {
+    const compressed = await promisify(zlib.zstdCompress)(input, flushOnly(2 ** 32));
+    expect(frameContentSize(compressed)).toBe(4294967296n);
+  });
+
+  it("accepts 4 GiB and larger when streaming", async () => {
+    const encoder = zlib.createZstdCompress({ pledgedSrcSize: 2 ** 32 });
+    try {
+      const chunks: Buffer[] = [];
+      const { promise, resolve, reject } = Promise.withResolvers<void>();
+      encoder.on("data", chunk => chunks.push(chunk));
+      encoder.on("error", reject);
+      encoder.on("close", () => reject(new Error("encoder closed before the flush completed")));
+      encoder.write(input, err => err && reject(err));
+      encoder.flush(() => resolve());
+      await promise;
+      expect(frameContentSize(Buffer.concat(chunks))).toBe(4294967296n);
+    } finally {
+      encoder.destroy();
+    }
+  });
+
+  it.each([Number.MAX_SAFE_INTEGER + 1, -1, 1.5, Infinity, NaN])("rejects %p as pledgedSrcSize", pledgedSrcSize => {
+    expect(() => zlib.createZstdCompress({ pledgedSrcSize })).toThrow(
+      expect.objectContaining({
+        code: "ERR_OUT_OF_RANGE",
+        message: expect.stringContaining('The value of "pledgedSrcSize" is out of range.'),
+      }),
+    );
+  });
+});