diff --git a/cbor/_common_encode.ts b/cbor/_common_encode.ts
index b038bd86eb4a..ab8c299a67cd 100644
--- a/cbor/_common_encode.ts
+++ b/cbor/_common_encode.ts
@@ -1,7 +1,14 @@
 // Copyright 2018-2026 the Deno authors. MIT license.
 
 import { CborTag } from "./tag.ts";
-import type { CborType } from "./types.ts";
+import type { ReadonlyCborType } from "./types.ts";
+
+// Narrows to ReadonlyMap (which TS's instanceof Map doesn't do on its own).
+function isReadonlyMap(
+  x: unknown,
+): x is ReadonlyMap<ReadonlyCborType, ReadonlyCborType> {
+  return x instanceof Map;
+}
 
 function calcBytes(x: bigint): number {
   let bytes = 0;
@@ -20,7 +27,7 @@ function calcHeaderSize(x: number | bigint): number {
   return 9;
 }
 
-export function calcEncodingSize(x: CborType): number {
+export function calcEncodingSize(x: ReadonlyCborType): number {
   if (x == undefined || typeof x === "boolean") return 1;
   if (typeof x === "number") {
     return x % 1 === 0 ? calcHeaderSize(x < 0 ? -x - 1 : x) : 9;
@@ -46,7 +53,7 @@ export function calcEncodingSize(x: CborType): number {
     for (const y of x) size += calcEncodingSize(y);
     return size;
   }
-  if (x instanceof Map) {
+  if (isReadonlyMap(x)) {
     let size = 3 + calcHeaderSize(x.size);
     for (const y of x) size += calcEncodingSize(y[0]) + calcEncodingSize(y[1]);
     return size;
@@ -61,7 +68,7 @@ export function calcEncodingSize(x: CborType): number {
 }
 
 export function encode(
-  input: CborType,
+  input: ReadonlyCborType,
   output: Uint8Array,
   offset: number,
 ): number {
@@ -86,8 +93,9 @@ export function encode(
         return encodeDate(input, output, offset);
       } else if (input instanceof CborTag) {
         return encodeTag(input, output, offset);
-      } else if (input instanceof Map) return encodeMap(input, output, offset);
-      else if (input instanceof Array) {
+      } else if (isReadonlyMap(input)) {
+        return encodeMap(input, output, offset);
+      } else if (input instanceof Array) {
         return encodeArray(input, output, offset);
       } else return encodeObject(input, output, offset);
   }
@@ -202,7 +210,7 @@ function encodeString(
 }
 
 function encodeArray(
-  input: CborType[],
+  input: readonly ReadonlyCborType[],
   output: Uint8Array,
   offset: number,
 ): number {
@@ -212,7 +220,7 @@ function encodeArray(
 }
 
 function encodeObject(
-  input: { [k: string]: CborType },
+  input: { readonly [k: string]: ReadonlyCborType },
   output: Uint8Array,
   offset: number,
 ): number {
@@ -231,7 +239,7 @@ function encodeDate(input: Date, output: Uint8Array, offset: number): number {
 }
 
 function encodeTag(
-  input: CborTag<CborType>,
+  input: CborTag<ReadonlyCborType>,
   output: Uint8Array,
   offset: number,
 ): number {
@@ -255,7 +263,7 @@ function encodeTag(
 }
 
 function encodeMap(
-  input: Map<CborType, CborType>,
+  input: ReadonlyMap<ReadonlyCborType, ReadonlyCborType>,
   output: Uint8Array,
   offset: number,
 ): number {
diff --git a/cbor/encode_cbor.ts b/cbor/encode_cbor.ts
index 8483d3a410a9..3de64d8f7a26 100644
--- a/cbor/encode_cbor.ts
+++ b/cbor/encode_cbor.ts
@@ -1,11 +1,11 @@
 // Copyright 2018-2026 the Deno authors. MIT license.
 
 import { calcEncodingSize, encode } from "./_common_encode.ts";
-import type { CborType } from "./types.ts";
+import type { CborType, ReadonlyCborType } from "./types.ts";
 
 /**
- * Encodes a {@link CborType} value into a CBOR format represented as a
- * {@link Uint8Array}.
+ * Encodes a {@link CborType} or {@link ReadonlyCborType} value into a CBOR
+ * format represented as a {@link Uint8Array}.
  * [RFC 8949 - Concise Binary Object Representation (CBOR)](https://datatracker.ietf.org/doc/html/rfc8949)
  *
  * @example Usage
@@ -32,10 +32,11 @@ import type { CborType } from "./types.ts";
  * assertEquals(decodedMessage, rawMessage);
  * ```
  *
- * @param value The value to encode of type {@link CborType}.
+ * @param value The value to encode of type {@link CborType} or
+ *              {@link ReadonlyCborType}.
  * @returns A {@link Uint8Array} representing the encoded data.
  */
-export function encodeCbor(value: CborType): Uint8Array {
+export function encodeCbor(value: CborType | ReadonlyCborType): Uint8Array {
   const output = new Uint8Array(calcEncodingSize(value));
   const o = encode(value, output, 0);
   if (o !== output.length) return output.subarray(0, o);
diff --git a/cbor/encode_cbor_sequence.ts b/cbor/encode_cbor_sequence.ts
index 2a3f3661f8af..e9051976ad8e 100644
--- a/cbor/encode_cbor_sequence.ts
+++ b/cbor/encode_cbor_sequence.ts
@@ -1,11 +1,11 @@
 // Copyright 2018-2026 the Deno authors. MIT license.
 
 import { calcEncodingSize, encode } from "./_common_encode.ts";
-import type { CborType } from "./types.ts";
+import type { CborType, ReadonlyCborType } from "./types.ts";
 
 /**
- * Encodes an array of {@link CborType} values into a CBOR format sequence
- * represented as a {@link Uint8Array}.
+ * Encodes an array of {@link CborType} or {@link ReadonlyCborType} values
+ * into a CBOR format sequence represented as a {@link Uint8Array}.
  * [RFC 8949 - Concise Binary Object Representation (CBOR)](https://datatracker.ietf.org/doc/html/rfc8949)
  *
  * @example Usage
@@ -31,10 +31,13 @@ import type { CborType } from "./types.ts";
  * assertEquals(decodedMessage, rawMessage);
  * ```
  *
- * @param values An array of values to encode of type {@link CborType}
+ * @param values An array of values to encode, each of type {@link CborType}
+ *               or {@link ReadonlyCborType}.
  * @returns A {@link Uint8Array} representing the encoded data.
  */
-export function encodeCborSequence(values: CborType[]): Uint8Array {
+export function encodeCborSequence(
+  values: readonly (CborType | ReadonlyCborType)[],
+): Uint8Array {
   let o = 0;
   for (const value of values) o += calcEncodingSize(value);
   const output = new Uint8Array(o);
diff --git a/cbor/encode_cbor_sequence_test.ts b/cbor/encode_cbor_sequence_test.ts
index 24200c4264a7..425f5309c62e 100644
--- a/cbor/encode_cbor_sequence_test.ts
+++ b/cbor/encode_cbor_sequence_test.ts
@@ -2,6 +2,7 @@
 
 import { assertEquals } from "@std/assert";
 import { encodeCborSequence } from "./encode_cbor_sequence.ts";
+import type { CborType } from "./types.ts";
 
 Deno.test("encodeCborSequence() correctly encoding", () => {
   assertEquals(
@@ -9,3 +10,12 @@ Deno.test("encodeCborSequence() correctly encoding", () => {
     Uint8Array.from([0b000_00000, 0b000_00000]),
   );
 });
+
+Deno.test("encodeCborSequence() accepting readonly array input", () => {
+  const values = [1, "two", { three: 3n }] as const;
+  const mutable: CborType[] = [1, "two", { three: 3n }];
+  assertEquals(
+    encodeCborSequence(values),
+    encodeCborSequence(mutable),
+  );
+});
diff --git a/cbor/encode_cbor_test.ts b/cbor/encode_cbor_test.ts
index 0e71085be55d..4e408dd0e75f 100644
--- a/cbor/encode_cbor_test.ts
+++ b/cbor/encode_cbor_test.ts
@@ -498,3 +498,53 @@ Deno.test("encodeCbor() rejecting CborTag()", () => {
     `Cannot encode Tag Item: Tag Number (${num}) exceeds 2 ** 64 - 1`,
   );
 });
+
+Deno.test("encodeCbor() accepting `as const` (deeply readonly) input", () => {
+  const data = {
+    a: 1,
+    b: { c: 2n },
+    d: [3, { e: 4 }],
+  } as const;
+
+  assertEquals(encodeCbor(data), encodeCbor(structuredClone(data)));
+});
+
+Deno.test("encodeCbor() accepting readonly array literals", () => {
+  const tuple = ["hello", 42, { nested: "value" }] as const;
+  assertEquals(
+    encodeCbor(tuple),
+    encodeCbor(["hello", 42, { nested: "value" }]),
+  );
+});
+
+Deno.test("encodeCbor() accepting ReadonlyMap input", () => {
+  const map: ReadonlyMap<CborType, CborType> = new Map<CborType, CborType>([
+    [1, 2],
+    ["3", 4],
+    [[5], { a: 6 }],
+  ]);
+
+  assertEquals(
+    encodeCbor(map),
+    encodeCbor(
+      new Map<CborType, CborType>([
+        [1, 2],
+        ["3", 4],
+        [[5], { a: 6 }],
+      ]),
+    ),
+  );
+});
+
+Deno.test("encodeCbor() accepting readonly index signature", () => {
+  const obj: { readonly [k: string]: number } = { x: 1, y: 2, z: 3 };
+  assertEquals(encodeCbor(obj), encodeCbor({ x: 1, y: 2, z: 3 }));
+});
+
+Deno.test("encodeCbor() accepting CborTag with readonly content", () => {
+  const data = [1, { inner: 2 }] as const;
+  assertEquals(
+    encodeCbor(new CborTag(1, data)),
+    encodeCbor(new CborTag(1, structuredClone(data))),
+  );
+});
diff --git a/cbor/sequence_encoder_stream.ts b/cbor/sequence_encoder_stream.ts
index 990c57c4d166..9f59c5c4ff80 100644
--- a/cbor/sequence_encoder_stream.ts
+++ b/cbor/sequence_encoder_stream.ts
@@ -145,7 +145,9 @@ export class CborSequenceEncoderStream
     } else yield encodeCbor(x);
   }
 
-  async *#encodeArray(x: CborStreamInput[]): AsyncGenerator<Uint8Array> {
+  async *#encodeArray(
+    x: readonly CborStreamInput[],
+  ): AsyncGenerator<Uint8Array> {
     if (x.length < 24) yield new Uint8Array([0b100_00000 + x.length]);
     else if (x.length < 2 ** 8) yield new Uint8Array([0b100_11000, x.length]);
     else if (x.length < 2 ** 16) {
@@ -162,7 +164,7 @@ export class CborSequenceEncoderStream
   }
 
   async *#encodeObject(
-    x: { [k: string]: CborStreamInput },
+    x: { readonly [k: string]: CborStreamInput },
   ): AsyncGenerator<Uint8Array> {
     const len = Object.keys(x).length;
     if (len < 24) yield new Uint8Array([0b101_00000 + len]);
diff --git a/cbor/tag.ts b/cbor/tag.ts
index 497ba62482cf..ee3b7f4faf41 100644
--- a/cbor/tag.ts
+++ b/cbor/tag.ts
@@ -1,6 +1,11 @@
 // Copyright 2018-2026 the Deno authors. MIT license.
 
-import type { CborStreamInput, CborStreamOutput, CborType } from "./types.ts";
+import type {
+  CborStreamInput,
+  CborStreamOutput,
+  CborType,
+  ReadonlyCborType,
+} from "./types.ts";
 
 /**
  * Represents a CBOR tag, which pairs a tag number with content, used to convey
@@ -30,9 +35,12 @@ import type { CborStreamInput, CborStreamOutput, CborType } from "./types.ts";
  * ```
  *
  * @typeParam T The type of the tag's content, which can be a
- * {@link CborType}, {@link CborStreamInput}, or {@link CborStreamOutput}.
+ * {@link CborType}, {@link ReadonlyCborType}, {@link CborStreamInput}, or
+ * {@link CborStreamOutput}.
  */
-export class CborTag<T extends CborType | CborStreamInput | CborStreamOutput> {
+export class CborTag<
+  T extends CborType | ReadonlyCborType | CborStreamInput | CborStreamOutput,
+> {
   /**
    * A {@link number} or {@link bigint} representing the CBOR tag number, used
    * to identify the type of the tagged content.
diff --git a/cbor/types.ts b/cbor/types.ts
index d75b773650a9..dc7d83cc03b4 100644
--- a/cbor/types.ts
+++ b/cbor/types.ts
@@ -28,6 +28,9 @@ export type CborPrimitiveType =
  * This type specifies the encodable and decodable values for
  * {@link encodeCbor}, {@link decodeCbor}, {@link encodeCborSequence}, and
  * {@link decodeCborSequence}.
+ *
+ * The encoder functions also accept {@link ReadonlyCborType}, which permits
+ * `readonly` arrays, `ReadonlyMap`, and `readonly` index signatures.
  */
 export type CborType =
   | CborPrimitiveType
@@ -38,15 +41,45 @@ export type CborType =
     [k: string]: CborType;
   };
 
+/**
+ * Readonly variant of {@link CborType} accepted by {@link encodeCbor} and
+ * {@link encodeCborSequence}. Lets you pass `as const` literals,
+ * `ReadonlyMap`s, and frozen arrays without casting.
+ *
+ * @example Usage
+ * ```ts
+ * import { encodeCbor } from "@std/cbor";
+ *
+ * const data = {
+ *   a: 1,
+ *   b: { c: 2n },
+ *   d: [3, { e: 4 }],
+ * } as const;
+ *
+ * encodeCbor(data);
+ * ```
+ */
+export type ReadonlyCborType =
+  | CborPrimitiveType
+  | CborTag<ReadonlyCborType>
+  | ReadonlyMap<ReadonlyCborType, ReadonlyCborType>
+  | readonly ReadonlyCborType[]
+  | {
+    readonly [k: string]: ReadonlyCborType;
+  };
+
 /**
  * Specifies the encodable value types for the {@link CborSequenceEncoderStream}
  * and {@link CborArrayEncoderStream}.
+ *
+ * Arrays and index signatures are `readonly` so `as const` literals work
+ * without casting.
  */
 export type CborStreamInput =
   | CborPrimitiveType
   | CborTag<CborStreamInput>
-  | CborStreamInput[]
-  | { [k: string]: CborStreamInput }
+  | readonly CborStreamInput[]
+  | { readonly [k: string]: CborStreamInput }
   | CborByteEncoderStream
   | CborTextEncoderStream
   | CborArrayEncoderStream