diff --git a/README.md b/README.md
index 09cb444..74f4b29 100644
--- a/README.md
+++ b/README.md
@@ -178,7 +178,7 @@ More information about enforcement levels can be found in [the specification](ht
 Cookie accepts `encode` or `decode` options for processing a [cookie-value](https://datatracker.ietf.org/doc/html/rfc6265#section-4.1.1).
 Since the value of a cookie has a limited character set (and must be a simple string), these functions are used to transform values into strings suitable for a cookies value.
 
-The default `encode` function is the global `encodeURIComponent`.
+The default `encode` function preserves roundtrip-safe cookie-octet values and uses the global `encodeURIComponent` otherwise.
 
 The default `decode` function is the global `decodeURIComponent`, wrapped in a `try..catch`. If an error
 is thrown it will return the cookie's original value. If you provide your own encode/decode
diff --git a/src/index.ts b/src/index.ts
index a95a2e9..b2f233e 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -68,6 +68,11 @@ const pathValueRegExp = /^[\u0020-\u003A\u003D-\u007E]*$/;
  */
 const maxAgeRegExp = /^-?\d+$/;
 
+/**
+ * RegExp to match RFC 6265 cookie-octet values (without % to preserve roundtrip) that need no URL encoding.
+ */
+const cookieOctetRegExp = /^[!#$&'()*+\-.\/0-9:<=>?@A-Z[\]\^_`a-z{|}~]*$/;
+
 const __toString = Object.prototype.toString;
 
 const NullObject = /* @__PURE__ */ (() => {
@@ -144,8 +149,7 @@ export interface StringifyOptions {
    * Specifies a function that will be used to encode a [cookie-value](https://datatracker.ietf.org/doc/html/rfc6265#section-4.1.1).
    * Since value of a cookie has a limited character set (and must be a simple string), this function can be used to encode
    * a value into a string suited for a cookie's value, and should mirror `decode` when parsing.
-   *
-   * @default encodeURIComponent
+   * The default function preserves roundtrip-safe cookie-octet values and uses `encodeURIComponent` otherwise.
    */
   encode?: (str: string) => string;
 }
@@ -157,7 +161,7 @@ export function stringifyCookie(
   cookie: Cookies,
   options?: StringifyOptions,
 ): string {
-  const enc = options?.encode || encodeURIComponent;
+  const enc = options?.encode || defaultEncode;
   const keys = Object.keys(cookie);
   let str = "";
 
@@ -298,7 +302,7 @@ export function stringifySetCookie(
       ? _name
       : { ..._opts, name: _name, value: String(_val) };
   const options = typeof _val === "object" ? _val : _opts;
-  const enc = options?.encode || encodeURIComponent;
+  const enc = options?.encode || defaultEncode;
 
   if (!cookieNameRegExp.test(cookie.name)) {
     throw new TypeError(`argument name is invalid: ${cookie.name}`);
@@ -534,6 +538,13 @@ function decode(str: string): string {
   }
 }
 
+/**
+ * URL-encode string value. Optimized to skip native call for roundtrip-safe cookie-octet values.
+ */
+function defaultEncode(str: string): string {
+  return cookieOctetRegExp.test(str) ? str : encodeURIComponent(str);
+}
+
 /**
  * Determine if value is a Date.
  */
diff --git a/src/stringify-cookie.bench.ts b/src/stringify-cookie.bench.ts
index 9da3464..f078ab7 100644
--- a/src/stringify-cookie.bench.ts
+++ b/src/stringify-cookie.bench.ts
@@ -10,6 +10,14 @@ describe("cookie.stringifyCookie", () => {
     cookie.stringifyCookie({ foo: "bar" });
   });
 
+  bench("rfc cookie-octets", () => {
+    cookie.stringifyCookie({ foo: "a=b+c/d?x~" });
+  });
+
+  bench("encode", () => {
+    cookie.stringifyCookie({ foo: "bar baz;%" });
+  });
+
   bench("undefined values", () => {
     cookie.stringifyCookie({
       foo: "bar",
@@ -19,6 +27,14 @@ describe("cookie.stringifyCookie", () => {
     });
   });
 
+  bench("mixed encode", () => {
+    cookie.stringifyCookie({
+      foo: "bar",
+      baz: "quux zap",
+      qux: "quux",
+    });
+  });
+
   const cookies10 = genCookies(10);
   bench("10 cookies", () => {
     cookie.stringifyCookie(cookies10);
diff --git a/src/stringify-cookie.spec.ts b/src/stringify-cookie.spec.ts
index 9edcfde..0e4c634 100644
--- a/src/stringify-cookie.spec.ts
+++ b/src/stringify-cookie.spec.ts
@@ -1,6 +1,38 @@
 import { describe, expect, it } from "vitest";
 import { stringifyCookie, parseCookie } from "./index.js";
 
+const cookieOctets =
+  "!#$&'()*+-./0123456789:<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[]" +
+  "^_`abcdefghijklmnopqrstuvwxyz{|}~";
+
+const bmpEncodingCases: Array<[string, string, string]> = [];
+
+for (let code = 0; code <= 0xffff; code++) {
+  // encodeURIComponent throws on unpaired surrogates.
+  if (code >= 0xd800 && code <= 0xdfff) continue;
+
+  const value = String.fromCharCode(code);
+  const encoded = cookieOctets.includes(value)
+    ? value
+    : encodeURIComponent(value);
+
+  bmpEncodingCases.push([
+    `U+${code.toString(16).toUpperCase().padStart(4, "0")}`,
+    value,
+    `key=${encoded}`,
+  ]);
+}
+
+const astralEncodingCases: Array<[string, string, string]> = [
+  "😄",
+  "𝌆",
+  "𠜎",
+].map((value) => [
+  `U+${value.codePointAt(0)!.toString(16).toUpperCase()}`,
+  value,
+  `key=${encodeURIComponent(value)}`,
+]);
+
 describe("cookie.stringifyCookie", () => {
   it("should stringify object", () => {
     expect(stringifyCookie({ key: "value" })).toEqual("key=value");
@@ -28,14 +60,39 @@ describe("cookie.stringifyCookie", () => {
     expect(stringifyCookie({ a: "", b: "" })).toEqual("a=; b=");
   });
 
-  it("should URL-encode values by default", () => {
+  it("should encode values with non-cookie-octet chars by default", () => {
     expect(stringifyCookie({ foo: "bar baz" })).toEqual("foo=bar%20baz");
-    expect(stringifyCookie({ foo: "a=b" })).toEqual("foo=a%3Db");
     expect(stringifyCookie({ foo: "hello;world" })).toEqual(
       "foo=hello%3Bworld",
     );
+    expect(stringifyCookie({ foo: 'hello"world' })).toEqual(
+      "foo=hello%22world",
+    );
+    expect(stringifyCookie({ foo: "foo,bar" })).toEqual("foo=foo%2Cbar");
+    expect(stringifyCookie({ foo: "foo\\bar" })).toEqual("foo=foo%5Cbar");
+    expect(stringifyCookie({ foo: "100%" })).toEqual("foo=100%25");
+  });
+
+  it("should pass through roundtrip-safe cookie-octet values without encoding", () => {
+    const value = cookieOctets;
+
+    expect(stringifyCookie({ foo: value })).toEqual(`foo=${value}`);
   });
 
+  it.each(bmpEncodingCases)(
+    "should match default encoding for BMP char %s",
+    (_name, value, expected) => {
+      expect(stringifyCookie({ key: value })).toEqual(expected);
+    },
+  );
+
+  it.each(astralEncodingCases)(
+    "should match default encoding for astral char %s",
+    (_name, value, expected) => {
+      expect(stringifyCookie({ key: value })).toEqual(expected);
+    },
+  );
+
   it("should error on invalid keys", () => {
     expect(() => stringifyCookie({ "test=": "" })).toThrow(
       /cookie name is invalid/,
@@ -114,6 +171,14 @@ describe("cookie.stringifyCookie", () => {
       expect(parseCookie(str)).toEqual(cookies);
     });
 
+    it("should roundtrip percent-encoded-looking values", () => {
+      const cookies = { foo: "%20" };
+      const str = stringifyCookie(cookies);
+
+      expect(str).toEqual("foo=%2520");
+      expect(parseCookie(str)).toEqual(cookies);
+    });
+
     it("should roundtrip empty values", () => {
       const cookies = { a: "", b: "value" };
       const str = stringifyCookie(cookies);
diff --git a/src/stringify-set-cookie.spec.ts b/src/stringify-set-cookie.spec.ts
index 92a56ea..e9c2983 100644
--- a/src/stringify-set-cookie.spec.ts
+++ b/src/stringify-set-cookie.spec.ts
@@ -1,6 +1,10 @@
 import { describe, it, expect } from "vitest";
 import * as cookie from "./index.js";
 
+const cookieOctets =
+  "!#$&'()*+-./0123456789:<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[]" +
+  "^_`abcdefghijklmnopqrstuvwxyz{|}~";
+
 describe("cookie.stringifySetCookie", function () {
   it("should have backward compatible export", function () {
     expect(cookie.serialize).toBe(cookie.stringifySetCookie);
@@ -10,10 +14,17 @@ describe("cookie.stringifySetCookie", function () {
     expect(cookie.stringifySetCookie("foo", "bar")).toEqual("foo=bar");
   });
 
-  it("should URL-encode value", function () {
+  it("should encode values with non-cookie-octet chars", function () {
     expect(cookie.stringifySetCookie("foo", "bar +baz")).toEqual(
       "foo=bar%20%2Bbaz",
     );
+    expect(cookie.stringifySetCookie("foo", "100%")).toEqual("foo=100%25");
+  });
+
+  it("should pass through roundtrip-safe cookie-octet values without encoding", function () {
+    const value = cookieOctets;
+
+    expect(cookie.stringifySetCookie("foo", value)).toEqual(`foo=${value}`);
   });
 
   it("should serialize empty value", function () {