diff --git a/registry/client/client.go b/registry/client/client.go
index d5a2f31..a096d44 100644
--- a/registry/client/client.go
+++ b/registry/client/client.go
@@ -596,6 +596,11 @@ func (c *Client) RegisterWithKeyOpts(o RegisterOpts) (map[string]interface{}, er
 	if o.RelayOnly {
 		msg["relay_only"] = true
 	}
+	if o.PublicKey != "" {
+		if sig, err := c.sign(fmt.Sprintf("register:%s:%s", o.ListenAddr, o.PublicKey)); err == nil {
+			msg["signature"] = sig
+		}
+	}
 	return c.Send(msg)
 }
 
diff --git a/registry/client/zz_client_register_signature_test.go b/registry/client/zz_client_register_signature_test.go
new file mode 100644
index 0000000..96ceccf
--- /dev/null
+++ b/registry/client/zz_client_register_signature_test.go
@@ -0,0 +1,105 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+package client
+
+import (
+	"crypto/ed25519"
+	"crypto/rand"
+	"encoding/base64"
+	"fmt"
+	"testing"
+)
+
+func TestRegisterWithKeyOptsCarriesProofOfPossession(t *testing.T) {
+	t.Parallel()
+	pub, priv, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatalf("keygen: %v", err)
+	}
+	pubB64 := base64.StdEncoding.EncodeToString(pub)
+
+	srv := newFakeJSONServer(t, echoHandler())
+	defer srv.close()
+	c, _ := Dial(srv.addr())
+	defer c.Close()
+
+	c.SetSigner(func(challenge string) string {
+		return base64.StdEncoding.EncodeToString(ed25519.Sign(priv, []byte(challenge)))
+	})
+
+	const listenAddr = "1.2.3.4:4000"
+	resp, err := c.RegisterWithKeyOpts(RegisterOpts{
+		ListenAddr: listenAddr,
+		PublicKey:  pubB64,
+	})
+	if err != nil {
+		t.Fatalf("register: %v", err)
+	}
+	echo, _ := resp["echo"].(map[string]interface{})
+	sigB64, ok := echo["signature"].(string)
+	if !ok || sigB64 == "" {
+		t.Fatalf("register message carries no signature: %#v", echo)
+	}
+	sig, err := base64.StdEncoding.DecodeString(sigB64)
+	if err != nil {
+		t.Fatalf("signature not base64: %v", err)
+	}
+	challenge := fmt.Sprintf("register:%s:%s", listenAddr, pubB64)
+	if !ed25519.Verify(pub, []byte(challenge), sig) {
+		t.Fatalf("signature does not verify against submitted public_key for %q", challenge)
+	}
+}
+
+func TestRegisterWithKeyReRegistrationBindsPublicKey(t *testing.T) {
+	t.Parallel()
+	pub, priv, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatalf("keygen: %v", err)
+	}
+	pubB64 := base64.StdEncoding.EncodeToString(pub)
+
+	srv := newFakeJSONServer(t, echoHandler())
+	defer srv.close()
+	c, _ := Dial(srv.addr())
+	defer c.Close()
+
+	c.SetSigner(func(challenge string) string {
+		return base64.StdEncoding.EncodeToString(ed25519.Sign(priv, []byte(challenge)))
+	})
+
+	const listenAddr = "x:1"
+	resp, err := c.RegisterWithKey(listenAddr, pubB64, "bob", []string{"10.0.0.1:80"}, "v1.2.3")
+	if err != nil {
+		t.Fatalf("register: %v", err)
+	}
+	echo, _ := resp["echo"].(map[string]interface{})
+	sigB64, _ := echo["signature"].(string)
+	sig, err := base64.StdEncoding.DecodeString(sigB64)
+	if err != nil || sigB64 == "" {
+		t.Fatalf("re-registration carries no verifiable signature: %q (%v)", sigB64, err)
+	}
+	challenge := fmt.Sprintf("register:%s:%s", listenAddr, pubB64)
+	if !ed25519.Verify(pub, []byte(challenge), sig) {
+		t.Fatalf("re-registration signature does not bind submitted public_key")
+	}
+}
+
+func TestRegisterWithKeyOptsNoSignerOmitsSignature(t *testing.T) {
+	t.Parallel()
+	srv := newFakeJSONServer(t, echoHandler())
+	defer srv.close()
+	c, _ := Dial(srv.addr())
+	defer c.Close()
+
+	resp, err := c.RegisterWithKeyOpts(RegisterOpts{ListenAddr: "x:1", PublicKey: "PUB=="})
+	if err != nil {
+		t.Fatalf("register without signer should still succeed: %v", err)
+	}
+	echo, _ := resp["echo"].(map[string]interface{})
+	if _, ok := echo["signature"]; ok {
+		t.Fatalf("signature must be omitted when no signer is configured (backward compatibility)")
+	}
+	if got, _ := echo["public_key"].(string); got != "PUB==" {
+		t.Fatalf("public_key: %q", got)
+	}
+}