diff --git a/hsmd/capabilities.h b/hsmd/capabilities.h
index 565356b40..3a306e778 100644
--- a/hsmd/capabilities.h
+++ b/hsmd/capabilities.h
@@ -6,6 +6,7 @@
 #define HSM_CAP_SIGN_ONCHAIN_TX 4
 #define HSM_CAP_COMMITMENT_POINT 8
 #define HSM_CAP_SIGN_REMOTE_TX 16
+#define HSM_CAP_SIGN_CLOSING_TX 32
 
 #define HSM_CAP_MASTER 1024
 #endif /* LIGHTNING_HSMD_CAPABILITIES_H */
diff --git a/hsmd/hsm.c b/hsmd/hsm.c
index 214c5b8cd..02e6dc4ea 100644
--- a/hsmd/hsm.c
+++ b/hsmd/hsm.c
@@ -683,6 +683,55 @@ fail:
 	return io_close(conn);
 }
 
+static struct io_plan *handle_sign_mutual_close_tx(struct io_conn *conn,
+						   struct client *c)
+{
+	struct daemon_conn *dc = &c->dc;
+	struct secret channel_seed;
+	struct bitcoin_tx *tx;
+	struct pubkey remote_funding_pubkey, local_funding_pubkey;
+	secp256k1_ecdsa_signature sig;
+	struct secrets secrets;
+	u64 funding_amount;
+	const u8 *funding_wscript;
+
+	if (!fromwire_hsm_sign_mutual_close_tx(tmpctx, dc->msg_in,
+					       &tx,
+					       &remote_funding_pubkey,
+					       &funding_amount)) {
+		status_broken("bad hsm_sign_htlc_mutual_close_tx for client %s",
+			      type_to_string(tmpctx, struct pubkey, &c->id));
+		goto fail;
+	}
+
+	/* FIXME: We should know dust level, decent fee range and
+	 * balances, and final_keyindex, and thus be able to check tx
+	 * outputs! */
+	get_channel_seed(&c->id, c->dbid, &channel_seed);
+	derive_basepoints(&channel_seed,
+			  &local_funding_pubkey, NULL, &secrets, NULL);
+
+	funding_wscript = bitcoin_redeem_2of2(tmpctx,
+					      &local_funding_pubkey,
+					      &remote_funding_pubkey);
+	/* Need input amount for signing */
+	tx->input[0].amount = tal_dup(tx->input, u64, &funding_amount);
+	sign_tx_input(tx, 0, NULL, funding_wscript,
+		      &secrets.funding_privkey,
+		      &local_funding_pubkey,
+		      &sig);
+
+	daemon_conn_send(dc, take(towire_hsm_sign_tx_reply(NULL, &sig)));
+	return daemon_conn_read_next(conn, dc);
+
+fail:
+	daemon_conn_send(c->master,
+			 take(towire_hsmstatus_client_bad_request(NULL,
+							  &c->id,
+							  dc->msg_in)));
+	return io_close(conn);
+}
+
 static bool check_client_capabilities(struct client *client,
 				      enum hsm_client_wire_type t)
 {
@@ -708,6 +757,9 @@ static bool check_client_capabilities(struct client *client,
 	case WIRE_HSM_SIGN_REMOTE_HTLC_TX:
 		return (client->capabilities & HSM_CAP_SIGN_REMOTE_TX) != 0;
 
+	case WIRE_HSM_SIGN_MUTUAL_CLOSE_TX:
+		return (client->capabilities & HSM_CAP_SIGN_CLOSING_TX) != 0;
+
 	case WIRE_HSM_INIT:
 	case WIRE_HSM_CLIENT_HSMFD:
 	case WIRE_HSM_SIGN_FUNDING:
@@ -812,6 +864,9 @@ static struct io_plan *handle_client(struct io_conn *conn,
 	case WIRE_HSM_SIGN_REMOTE_HTLC_TX:
 		return handle_sign_remote_htlc_tx(conn, c);
 
+	case WIRE_HSM_SIGN_MUTUAL_CLOSE_TX:
+		return handle_sign_mutual_close_tx(conn, c);
+
 	case WIRE_HSM_ECDH_RESP:
 	case WIRE_HSM_CANNOUNCEMENT_SIG_REPLY:
 	case WIRE_HSM_CUPDATE_SIG_REPLY:
diff --git a/hsmd/hsm_client_wire_csv b/hsmd/hsm_client_wire_csv
index 1de135a12..1757caf9f 100644
--- a/hsmd/hsm_client_wire_csv
+++ b/hsmd/hsm_client_wire_csv
@@ -149,6 +149,12 @@ hsm_sign_remote_htlc_tx,,wscript,len*u8
 hsm_sign_remote_htlc_tx,,amounts_satoshi,u64
 hsm_sign_remote_htlc_tx,,remote_per_commit_point,struct pubkey
 
+# closingd asks HSM to sign mutual close tx.
+hsm_sign_mutual_close_tx,21
+hsm_sign_mutual_close_tx,,tx,struct bitcoin_tx
+hsm_sign_mutual_close_tx,,remote_funding_key,struct pubkey
+hsm_sign_mutual_close_tx,,funding_amount,u64
+
 # Reply for all the above requests.
 hsm_sign_tx_reply,112
 hsm_sign_tx_reply,,sig,secp256k1_ecdsa_signature