diff --git a/cmd/boulder-mtca/main.go b/cmd/boulder-mtca/main.go
index 32f33f75f9d..45bd17b8a5d 100644
--- a/cmd/boulder-mtca/main.go
+++ b/cmd/boulder-mtca/main.go
@@ -4,9 +4,13 @@ package notmain
 
 import (
 	"context"
+	"crypto/mldsa"
+	"crypto/x509"
 	"database/sql"
+	"encoding/pem"
 	"errors"
 	"flag"
+	"fmt"
 	"os"
 	"sync"
 	"time"
@@ -50,12 +54,37 @@ type Config struct {
 
 		// SequencingPeriod controls how frequently the MTCA sequences a batch and signs a checkpoint.
 		SequencingPeriod config.Duration `validate:"required"`
+
+		// Mirror identifies the mirror whose cosignatures must be verified
+		// before the MTCA serves them as part of a checkpoint.
+		Mirror cmd.MirrorConfig `validate:"required"`
 	}
 
 	Syslog        cmd.SyslogConfig
 	OpenTelemetry cmd.OpenTelemetryConfig
 }
 
+// loadMLDSAPublicKey reads a PEM-encoded PKIX ML-DSA-44 public key.
+func loadMLDSAPublicKey(filename string) (*mldsa.PublicKey, error) {
+	data, err := os.ReadFile(filename)
+	if err != nil {
+		return nil, err
+	}
+	block, _ := pem.Decode(data)
+	if block == nil || block.Type != "PUBLIC KEY" {
+		return nil, fmt.Errorf("no PUBLIC KEY PEM block in %s", filename)
+	}
+	parsed, err := x509.ParsePKIXPublicKey(block.Bytes)
+	if err != nil {
+		return nil, err
+	}
+	pubKey, ok := parsed.(*mldsa.PublicKey)
+	if !ok {
+		return nil, fmt.Errorf("key in %s is %T, must be ML-DSA-44", filename, parsed)
+	}
+	return pubKey, nil
+}
+
 func main() {
 	grpcAddr := flag.String("addr", "", "gRPC listen address override")
 	debugAddr := flag.String("debug-addr", "", "Debug server address override")
@@ -117,6 +146,9 @@ func main() {
 	s3c, err := bs3.FromConfig(c.MTCA.S3, logger)
 	cmd.FailOnError(err, "Loading S3 config")
 
+	mirrorPublicKey, err := loadMLDSAPublicKey(c.MTCA.Mirror.PublicKeyFile)
+	cmd.FailOnError(err, "Loading mirror public key")
+
 	mtcaImpl, err := mtca.New(
 		issuer,
 		profiles,
@@ -124,6 +156,8 @@ func main() {
 		c.MTCA.SequencingPeriod.Duration,
 		dbMap,
 		s3c,
+		c.MTCA.Mirror.ID,
+		mirrorPublicKey,
 		logger,
 		clk)
 	cmd.FailOnError(err, "Building MTCA")
diff --git a/mtca/mtca.go b/mtca/mtca.go
index e7f263aac75..fd8361e9362 100644
--- a/mtca/mtca.go
+++ b/mtca/mtca.go
@@ -27,6 +27,7 @@ import (
 	"github.com/letsencrypt/boulder/issuance"
 	blog "github.com/letsencrypt/boulder/log"
 	mtcapb "github.com/letsencrypt/boulder/mtca/proto"
+	"github.com/letsencrypt/boulder/trees/checkpoint"
 	"github.com/letsencrypt/boulder/trees/cosignature"
 	"github.com/letsencrypt/boulder/trees/entry"
 	"github.com/letsencrypt/boulder/trees/issuancelog"
@@ -47,6 +48,8 @@ func New(
 	sequencingPeriod time.Duration,
 	dbMap *borp.DbMap,
 	s3c simpleS3,
+	mirrorID string,
+	mirrorPublicKey *mldsa.PublicKey,
 	logger blog.Logger,
 	clk clock.Clock,
 ) (*mtca, error) {
@@ -66,12 +69,14 @@ func New(
 		issuer:   issuer,
 		profiles: profiles,
 		logID:    logID,
+		mirrorID: mirrorID,
 		pool:     &pool{maxSize: 100},
 
 		sequencingPeriod: sequencingPeriod,
 
 		db:  initDB(dbMap),
 		s3c: s3c,
+
 		log: logger,
 		clk: clk,
 	}
@@ -92,6 +97,12 @@ func New(
 	}
 	m.verifier = verifier
 
+	mirrorVerifier, err := cosignature.NewVerifier(mirrorID, mirrorPublicKey)
+	if err != nil {
+		return nil, fmt.Errorf("creating mirror verifier: %s", err)
+	}
+	m.mirrorVerifier = mirrorVerifier
+
 	return m, nil
 }
 
@@ -104,6 +115,11 @@ type mtca struct {
 	cosigner *cosignature.Cosigner
 	verifier *cosignature.Verifier
 
+	mirrorID       string
+	mirrorVerifier *cosignature.Verifier
+
+	servedCheckpointID int64
+
 	pool *pool
 
 	// frontier contains all the tiles on the right edge of the tree.
@@ -407,9 +423,10 @@ func (m *mtca) Loop(ctx context.Context) {
 }
 
 // sequence takes all entries from the pool, simulates writing them to tile storage, signs
-// and stores a new checkpoint, and notifies waiting RPCs.
+// and stores a new checkpoint, and notifies waiting RPCs. It also serves the latest
+// checkpoint's cosigned note once the mirror signature arrives, when the pool is empty.
 //
-// If the pool is empty, nothing happens.
+// If the pool is empty, no sequencing happens.
 // If the pool is non-empty, but the previous checkpoint doesn't have a mirror signature,
 // returns an error that wraps ErrCheckpointNotReady (without taking entries from the pool).
 // This is expected to be a common occurrence.
@@ -423,15 +440,22 @@ func (m *mtca) sequence(ctx context.Context) error {
 		return fmt.Errorf("call mtca.Preflight() before sequencing")
 	}
 
-	if m.pool.len() == 0 {
-		return nil
-	}
-
 	latest, err := m.latestCheckpoint(ctx)
 	if err != nil {
 		return err
 	}
 
+	if latest.mirrored() {
+		err = m.serveCheckpoint(ctx, latest)
+		if err != nil {
+			return err
+		}
+	}
+
+	if m.pool.len() == 0 {
+		return nil
+	}
+
 	if !latest.mirrored() {
 		return fmt.Errorf("temporary: checkpoint ID %d (tree size %d): %w",
 			latest.ID, latest.TreeSize, ErrCheckpointNotReady)
@@ -571,10 +595,6 @@ func (m *mtca) sequence(ctx context.Context) error {
 	//
 	// TODO(#8902): This should include indefinite retries on error. We've committed to the
 	// tree hash by signing it, so nothing can make progress until we've published the tiles.
-	//
-	// Once we add publishing of checkpoints as signed notes, publication of the signed note
-	// should come after this flush succeeds, so monitors don't try to fetch tiles that aren't
-	// yet available.
 	err = m.frontier.Publish(ctx, m.s3c, m.logID.TilePrefix())
 	if err != nil {
 		return fmt.Errorf("publishing tiles: %s", err)
@@ -590,7 +610,7 @@ func (m *mtca) sequence(ctx context.Context) error {
 	return nil
 }
 
-// checkpoint represents the database storage of a checkpoint and associated signatures.
+// checkpointRow represents the database storage of a checkpoint and associated signatures.
 //
 // For signing, the TreeSize and RootHash fields are incorporated into a `cosigned.Message`.
 type checkpointRow struct {
@@ -683,3 +703,49 @@ func (m *mtca) signCheckpoint(c *checkpointRow) ([]byte, error) {
 	}
 	return cosignature.RawSignature(timestampedCosignature)
 }
+
+// serveCheckpoint writes latest's note, carrying the MTCA signature and a
+// mirror cosignature, to the checkpoint path in tile storage. Per CQRP: "In
+// order for landmarks to be served by Chrome's Landmark Service, all
+// checkpoints MUST be served with a minimum of 2 cosignatures. One of these
+// MUST be from the MTC CA Operator and one MUST be from a Mirroring Cosigner
+// recognized by Chrome and not operated by the MTC CA Operator."
+func (m *mtca) serveCheckpoint(ctx context.Context, latest *checkpointRow) error {
+	if latest.ID == m.servedCheckpointID {
+		return nil
+	}
+	if len(latest.RootHash) != tlog.HashSize {
+		return fmt.Errorf("checkpoint %d root hash is %d bytes, want %d", latest.ID, len(latest.RootHash), tlog.HashSize)
+	}
+
+	// Verify the MTCA cosignature and produce its signature line.
+	tree := tlog.Tree{N: latest.TreeSize, Hash: tlog.Hash(latest.RootHash)}
+	caCosignatureLine, err := m.verifier.SignatureLine(m.logID.Origin(), tree, latest.MTCASignature)
+	if err != nil {
+		return fmt.Errorf("checkpoint %d MTCA signature: %s", latest.ID, err)
+	}
+
+	// Verify the mirror cosignature and produce its signature line.
+	if latest.MirrorID != m.mirrorID {
+		return fmt.Errorf("checkpoint %d cosigned by mirror %q, want %q", latest.ID, latest.MirrorID, m.mirrorID)
+	}
+	mirrorCosignatureLine, err := m.mirrorVerifier.SignatureLine(m.logID.Origin(), tree, latest.MirrorSignature)
+	if err != nil {
+		return fmt.Errorf("checkpoint %d mirror cosignature: %s", latest.ID, err)
+	}
+
+	// Produce a note containing both cosgnatures.
+	cp := checkpoint.Checkpoint{Origin: m.logID.Origin(), Tree: tree}
+	note, err := cp.SignedNoteForServing(caCosignatureLine, mirrorCosignatureLine)
+	if err != nil {
+		return err
+	}
+
+	// Finally, write it to the checkpoint path for serving.
+	err = tiles.WriteCheckpoint(ctx, m.s3c, m.logID.TilePrefix(), note)
+	if err != nil {
+		return fmt.Errorf("serving checkpoint %d: %s", latest.ID, err)
+	}
+	m.servedCheckpointID = latest.ID
+	return nil
+}
diff --git a/mtca/mtca_test.go b/mtca/mtca_test.go
index ade9ba8f373..6c4ecd585d9 100644
--- a/mtca/mtca_test.go
+++ b/mtca/mtca_test.go
@@ -90,6 +90,10 @@ func setup() (*mtca, *bs3test.FakeS3, func(), error) {
 	}
 
 	fs3 := bs3test.New()
+	mirrorKey, err := mldsa.NewPrivateKey(mldsa.MLDSA44(), make([]byte, 32))
+	if err != nil {
+		return nil, nil, nil, err
+	}
 	mtca, err := New(
 		issuer,
 		map[string]*issuance.Profile{"mtcExample": profile},
@@ -97,6 +101,8 @@ func setup() (*mtca, *bs3test.FakeS3, func(), error) {
 		100*time.Millisecond,
 		dbMap,
 		fs3,
+		"32473.9",
+		mirrorKey.PublicKey(),
 		logger,
 		clk)
 	if err != nil {
@@ -479,6 +485,13 @@ func TestSequenceStorageFailure(t *testing.T) {
 	t.Cleanup(cleanup)
 	mirrorCosign(t, mtca)
 
+	// Serve the cosigned checkpoint while storage is healthy, so the failure
+	// below lands on staging.
+	err = mtca.sequence(t.Context())
+	if err != nil {
+		t.Fatalf("sequencing to serve the cosigned checkpoint: %s", err)
+	}
+
 	mtca.pool.maxSize = 2
 	results := issueMany(t, mtca, 2)
 
diff --git a/test/config-next/mtca.json b/test/config-next/mtca.json
index 29b3ab723e8..732b25e48e3 100644
--- a/test/config-next/mtca.json
+++ b/test/config-next/mtca.json
@@ -5,6 +5,10 @@
 			"logNumber": 44
 		},
 		"sequencingPeriod": "100ms",
+		"mirror": {
+			"id": "32473.9",
+			"publicKeyFile": "test/certs/mtpki/mirror.pub.pem"
+		},
 		"db": {
 			"dbConnectFile": "test/secrets/mtca1_dburl"
 		},