diff --git a/trees/mirror/mirror.go b/trees/mirror/mirror.go
index f70900ef0fc..1be9f7df16f 100644
--- a/trees/mirror/mirror.go
+++ b/trees/mirror/mirror.go
@@ -55,6 +55,33 @@ func AddCheckpointRequest(oldSize int64, proof []tlog.Hash, signedCheckpoint []b
 	return b.Bytes(), nil
 }
 
+// SignSubtreeRequest builds the sign-subtree request body per
+// c2sp.org/tlog-witness. The proof must be a Subtree Consistency Proof from the
+// subtree to the checkpoint, empty when the subtree is the whole tree.
+func SignSubtreeRequest(start, end int64, subtreeHash tlog.Hash, proof []tlog.Hash, signedCheckpoint []byte) ([]byte, error) {
+	if start < 0 {
+		return nil, fmt.Errorf("negative subtree start %d", start)
+	}
+	if end <= start {
+		return nil, fmt.Errorf("subtree end %d not after start %d", end, start)
+	}
+	if len(proof) > maxProofLines {
+		return nil, fmt.Errorf("consistency proof has %d lines, want at most %d", len(proof), maxProofLines)
+	}
+	if len(signedCheckpoint) == 0 {
+		return nil, errors.New("empty checkpoint")
+	}
+	var b bytes.Buffer
+	fmt.Fprintf(&b, "subtree %d %d\n%s\n", start, end, subtreeHash)
+	for _, h := range proof {
+		b.WriteString(h.String())
+		b.WriteByte('\n')
+	}
+	b.WriteByte('\n')
+	b.Write(signedCheckpoint)
+	return b.Bytes(), nil
+}
+
 // parseDecimal parses an ASCII decimal string, accepting leading zeroes since
 // the specs require canonical decimals only in request bodies.
 func parseDecimal(s string) (int64, error) {
diff --git a/trees/mirror/mirror_test.go b/trees/mirror/mirror_test.go
index 4130d85ae32..bfb4c31b857 100644
--- a/trees/mirror/mirror_test.go
+++ b/trees/mirror/mirror_test.go
@@ -196,6 +196,42 @@ func TestEntryPackage(t *testing.T) {
 	}
 }
 
+func TestSignSubtreeRequest(t *testing.T) {
+	hash := mustHash(t, "CsUYapGGPo4dkMgIAUqom/Xajj7h2fB2MPA3j2jxq2I=")
+	proof := []tlog.Hash{mustHash(t, "PlRNCrwHpqhGrupue0L7gxbjbMiKA9temvuZZDDpkaw=")}
+	note := []byte("example.com/log\n512\n" + hash.String() + "\n\n— example.com/log AAAA\n")
+
+	body, err := SignSubtreeRequest(256, 512, hash, proof, note)
+	if err != nil {
+		t.Fatalf("SignSubtreeRequest: %s", err)
+	}
+	expect := "subtree 256 512\n" + hash.String() + "\n" + proof[0].String() + "\n\n" + string(note)
+	if string(body) != expect {
+		t.Errorf("SignSubtreeRequest = %q, want %q", body, expect)
+	}
+
+	_, err = SignSubtreeRequest(-1, 512, hash, nil, note)
+	if err == nil {
+		t.Error("SignSubtreeRequest with a negative start = nil error, want error")
+	}
+	_, err = SignSubtreeRequest(512, 256, hash, nil, note)
+	if err == nil {
+		t.Error("SignSubtreeRequest with end before start = nil error, want error")
+	}
+	_, err = SignSubtreeRequest(512, 512, hash, nil, note)
+	if err == nil {
+		t.Error("SignSubtreeRequest with an empty subtree = nil error, want error")
+	}
+	_, err = SignSubtreeRequest(0, 512, hash, make([]tlog.Hash, 64), note)
+	if err == nil {
+		t.Error("SignSubtreeRequest with 64 proof lines = nil error, want error")
+	}
+	_, err = SignSubtreeRequest(0, 512, hash, nil, nil)
+	if err == nil {
+		t.Error("SignSubtreeRequest with an empty checkpoint = nil error, want error")
+	}
+}
+
 func TestAddEntriesRequest(t *testing.T) {
 	origin := "oid/1.3.6.1.4.1.44947.4.1.0.44"
 	pkg := []byte{0, 1, 'x', 0}