diff --git a/RFC/0004-mldsa-peer-ids.md b/RFC/0004-mldsa-peer-ids.md
new file mode 100644
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@@ -0,0 +1,156 @@
+- Start Date: 2026-04-11
+- Related PRs: [js-libp2p/pull/3432](https://github.com/libp2p/js-libp2p/pull/3432)
+- Related Spec: [peer-ids/peer-ids.md](../peer-ids/peer-ids.md)
+
+# RFC 0004: ML-DSA Key Support for libp2p Peer Identities
+
+## Abstract
+
+This RFC proposes adding support for post-quantum ML-DSA identity keys to
+libp2p.
+
+It defines a new `KeyType` value for ML-DSA in libp2p key protobufs, a wire
+format for serializing ML-DSA public/private keys, and how peer IDs are derived
+from ML-DSA public keys.
+
+## Motivation
+
+libp2p peer identities are currently based on classical signature schemes (RSA,
+Ed25519, secp256k1, ECDSA).
+
+To prepare libp2p identity and signature systems for post-quantum migration, we
+need an interoperable encoding and verification story for ML-DSA keys.
+
+Without a shared specification:
+
+- Implementations may choose incompatible key encodings.
+- Cross-implementation interoperability is not guaranteed.
+- Future migration to hybrid or fully post-quantum deployments becomes harder.
+
+## Design
+
+### 1. `KeyType` extension
+
+The key protobuf enum in the peer-id spec is extended with:
+
+```protobuf
+enum KeyType {
+  RSA = 0;
+  Ed25519 = 1;
+  Secp256k1 = 2;
+  ECDSA = 3;
+  MLDSA = 4;
+}
+```
+
+### 2. ML-DSA parameter sets
+
+This RFC supports the three standardized ML-DSA parameter sets:
+
+- `ML-DSA-44`
+- `ML-DSA-65`
+- `ML-DSA-87`
+
+Parameter names and sizes are defined by [FIPS 204](#references).
+
+### 3. Key serialization in protobuf `Data`
+
+For `PublicKey` and `PrivateKey` messages where `Type = MLDSA`, the `Data`
+field is:
+
+```
+<variant-prefix><raw-key-bytes>
+```
+
+Where `variant-prefix` is one byte:
+
+- `0x01` = `ML-DSA-44`
+- `0x02` = `ML-DSA-65`
+- `0x03` = `ML-DSA-87`
+
+Raw key and signature lengths are fixed per ML-DSA variant:
+
+| Variant | Public key (bytes) | Expanded private key (bytes) | Signature (bytes) |
+| --- | ---: | ---: | ---: |
+| `ML-DSA-44` | 1312 | 2560 | 2420 |
+| `ML-DSA-65` | 1952 | 4032 | 3309 |
+| `ML-DSA-87` | 2592 | 4896 | 4627 |
+
+Implementations MUST reject malformed key payloads, including unknown
+`variant-prefix`, missing prefix, or length mismatch against the table above.
+
+### 4. Signature semantics
+
+ML-DSA signatures are generated and verified using the standard ML-DSA
+algorithm for the corresponding parameter set.
+
+- Implementations MUST sign the exact message bytes.
+- Implementations MUST NOT apply an additional pre-hash at the libp2p key API
+  boundary.
+- Implementations MUST verify signatures over the exact same message bytes and
+  ML-DSA parameter set.
+
+### 5. Peer ID derivation
+
+Peer ID derivation remains unchanged:
+
+1. Protobuf-encode `PublicKey` with `Type = MLDSA` and `Data` as specified
+  above.
+2. Compute peer ID multihash according to the existing rule:
+   - identity multihash if encoded key is `<= 42` bytes
+   - SHA-256 multihash otherwise
+
+Because all ML-DSA public key encodings are much larger than 42 bytes, ML-DSA
+peer IDs always use SHA-256 multihash.
+
+## Backward Compatibility
+
+- Existing peers and key types are unaffected.
+- Implementations that do not support `KeyType = MLDSA` will reject those keys
+  as unsupported.
+- Text and CID peer ID formats are unchanged.
+
+## Security Considerations
+
+- This RFC only defines identity key representation and signature verification
+  semantics.
+- It does not by itself provide hybrid authentication or downgrade resistance
+  between classical and post-quantum identities.
+- Deployments should continue evaluating algorithm maturity, implementation
+  quality, and runtime support in their target environments.
+
+## Implementation Status (Non-Normative)
+
+Current language support for ML-DSA is still evolving:
+
+- Node.js runtime support is experimental.
+- Browser WebCrypto support is unavailable.
+- Go has an internal implementation for ML-DSA but no public API as of Go 1.26.
+  - Worth noting: `crypto/mlkem` went public in Go 1.24, `crypto/mldsa` is in
+    proposal phase [golang/go#77626](https://github.com/golang/go/issues/77626).
+    For now, `github.com/cloudflare/circl/sign/mldsa` is available.
+
+This RFC specifies interoperability behavior independent of implementation
+maturity.
+Production deployments SHOULD evaluate runtime support, performance, and audit
+status before enabling ML-DSA identities.
+
+## Open Questions
+
+The following questions should be resolved before promoting this to a candidate
+recommendation:
+
+1. Default variant selection for new key generation (`ML-DSA-44`, `-65`, or
+  `-87`).
+2. Whether any libp2p subsystem should require dual-signature or hybrid
+   identity strategies.
+3. Whether to define mandatory test vectors in the peer-id spec for ML-DSA
+   encodings.
+4. Canonical private key format: should it be FIPS 204 expanded form
+   [multiformats/multicodec#399](https://github.com/multiformats/multicodec/pull/399),
+   W3C DI Quantum-Safe Cryptosuite)?
+
+## References
+
+- [FIPS 204] NIST, *Module-Lattice-Based Digital Signature Standard* (ML-DSA),
+  https://csrc.nist.gov/pubs/fips/204/final
diff --git a/peer-ids/peer-ids.md b/peer-ids/peer-ids.md
index 4daf38c44..d5c91bd5a 100644
--- a/peer-ids/peer-ids.md
+++ b/peer-ids/peer-ids.md
@@ -28,6 +28,7 @@ about maturity level and spec status.
         - [Ed25519](#ed25519)
         - [Secp256k1](#secp256k1)
         - [ECDSA](#ecdsa)
+        - [ML-DSA](#ml-dsa)
 - [Peer Ids](#peer-ids)
     - [String representation](#string-representation)
         - [Encoding](#encoding)
@@ -63,6 +64,7 @@ enum KeyType {
 	Ed25519 = 1;
 	Secp256k1 = 2;
 	ECDSA = 3;
+	MLDSA = 4;
 }
 
 message PublicKey {
@@ -110,16 +112,17 @@ The second is for generating peer ids; this is discussed in the section below.
 
 ### Key Types
 
-Four key types are supported:
+Five key types are supported:
  - RSA
  - Ed25519
  - Secp256k1
  - ECDSA
+ - ML-DSA
 
 Implementations MUST support Ed25519. Implementations SHOULD support RSA if they wish to
 interoperate with the mainline IPFS DHT and the default IPFS bootstrap nodes. Implementations MAY
-support Secp256k1 and ECDSA, but nodes using those keys may not be able to connect to all other
-nodes.
+support Secp256k1, ECDSA, and ML-DSA, but nodes using those keys may not be able to connect to all
+other nodes.
 
 In all cases, implementation MAY allow the user to enable/disable specific key
 types via configuration. Note that disabling support for compulsory key types
@@ -185,6 +188,33 @@ To sign a message, we hash the message with SHA 256, and then sign it with the
 [ECDSA standard algorithm](https://tools.ietf.org/html/rfc6979), then we encode
 it using [DER-encoded ASN.1.](https://wiki.openssl.org/index.php/DER)
 
+#### ML-DSA
+
+For `MLDSA` keys, the serialized `Data` field includes a one-byte variant prefix
+followed by raw key bytes:
+
+```
+<variant-prefix><raw-key-bytes>
+```
+
+Variant prefixes are:
+
+- `0x01`: ML-DSA-44
+- `0x02`: ML-DSA-65
+- `0x03`: ML-DSA-87
+
+Public/private key lengths are the standard lengths for each variant:
+
+- ML-DSA-44: public key `1312` bytes, private key `2560` bytes
+- ML-DSA-65: public key `1952` bytes, private key `4032` bytes
+- ML-DSA-87: public key `2592` bytes, private key `4896` bytes
+
+Implementations MUST reject malformed key encodings (unknown variant prefix,
+missing prefix, or key length mismatch).
+
+ML-DSA signatures follow the normal ML-DSA standard. Signatures are generated
+and verified over the exact message bytes.
+
 ### Test vectors
 
 The following test vectors are hex-encoded bytes of the above described protobuf encoding.
@@ -221,6 +251,9 @@ Specifically, to compute a peer ID of a key:
 5. If the length is greater than 42, then hash it using the SHA256
    multihash.
 
+Because ML-DSA public keys are larger than 42 bytes, ML-DSA peer IDs always use
+the SHA256 multihash.
+
 ### String representation
 
 There are two ways to represent peer IDs in text: as a raw