diff --git a/watchtower/wtmock/client_db.go b/watchtower/wtmock/client_db.go
new file mode 100644
index 00000000..54f9a697
--- /dev/null
+++ b/watchtower/wtmock/client_db.go
@@ -0,0 +1,223 @@
+package wtmock
+
+import (
+	"fmt"
+	"net"
+	"sync"
+	"sync/atomic"
+
+	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/lightningnetwork/lnd/watchtower/wtdb"
+)
+
+type towerPK [33]byte
+
+// ClientDB is a mock, in-memory database or testing the watchtower client
+// behavior.
+type ClientDB struct {
+	nextTowerID uint64 // to be used atomically
+
+	mu             sync.Mutex
+	sweepPkScripts map[lnwire.ChannelID][]byte
+	activeSessions map[wtdb.SessionID]*wtdb.ClientSession
+	towerIndex     map[towerPK]uint64
+	towers         map[uint64]*wtdb.Tower
+}
+
+// NewClientDB initializes a new mock ClientDB.
+func NewClientDB() *ClientDB {
+	return &ClientDB{
+		sweepPkScripts: make(map[lnwire.ChannelID][]byte),
+		activeSessions: make(map[wtdb.SessionID]*wtdb.ClientSession),
+		towerIndex:     make(map[towerPK]uint64),
+		towers:         make(map[uint64]*wtdb.Tower),
+	}
+}
+
+// CreateTower initializes a database entry with the given lightning address. If
+// the tower exists, the address is append to the list of all addresses used to
+// that tower previously.
+func (m *ClientDB) CreateTower(lnAddr *lnwire.NetAddress) (*wtdb.Tower, error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	var towerPubKey towerPK
+	copy(towerPubKey[:], lnAddr.IdentityKey.SerializeCompressed())
+
+	var tower *wtdb.Tower
+	towerID, ok := m.towerIndex[towerPubKey]
+	if ok {
+		tower = m.towers[towerID]
+		tower.AddAddress(lnAddr.Address)
+	} else {
+		towerID = atomic.AddUint64(&m.nextTowerID, 1)
+		tower = &wtdb.Tower{
+			ID:          towerID,
+			IdentityKey: lnAddr.IdentityKey,
+			Addresses:   []net.Addr{lnAddr.Address},
+		}
+	}
+
+	m.towerIndex[towerPubKey] = towerID
+	m.towers[towerID] = tower
+
+	return tower, nil
+}
+
+// MarkBackupIneligible records that particular commit height is ineligible for
+// backup. This allows the client to track which updates it should not attempt
+// to retry after startup.
+func (m *ClientDB) MarkBackupIneligible(chanID lnwire.ChannelID, commitHeight uint64) error {
+	return nil
+}
+
+// ListClientSessions returns the set of all client sessions known to the db.
+func (m *ClientDB) ListClientSessions() (map[wtdb.SessionID]*wtdb.ClientSession, error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	sessions := make(map[wtdb.SessionID]*wtdb.ClientSession)
+	for _, session := range m.activeSessions {
+		sessions[session.ID] = session
+	}
+
+	return sessions, nil
+}
+
+// CreateClientSession records a newly negotiated client session in the set of
+// active sessions. The session can be identified by its SessionID.
+func (m *ClientDB) CreateClientSession(session *wtdb.ClientSession) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	m.activeSessions[session.ID] = &wtdb.ClientSession{
+		TowerID:          session.TowerID,
+		Tower:            session.Tower,
+		SessionKeyDesc:   session.SessionKeyDesc,
+		SessionPrivKey:   session.SessionPrivKey,
+		ID:               session.ID,
+		Policy:           session.Policy,
+		SeqNum:           session.SeqNum,
+		TowerLastApplied: session.TowerLastApplied,
+		RewardPkScript:   session.RewardPkScript,
+		CommittedUpdates: make(map[uint16]*wtdb.CommittedUpdate),
+		AckedUpdates:     make(map[uint16]wtdb.BackupID),
+	}
+
+	return nil
+}
+
+// CommitUpdate persists the CommittedUpdate provided in the slot for (session,
+// seqNum). This allows the client to retransmit this update on startup.
+func (m *ClientDB) CommitUpdate(id *wtdb.SessionID, seqNum uint16,
+	update *wtdb.CommittedUpdate) (uint16, error) {
+
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	// Fail if session doesn't exist.
+	session, ok := m.activeSessions[*id]
+	if !ok {
+		return 0, wtdb.ErrClientSessionNotFound
+	}
+
+	// Check if an update has already been committed for this state.
+	dbUpdate, ok := session.CommittedUpdates[seqNum]
+	if ok {
+		// If the breach hint matches, we'll just return the last
+		// applied value so the client can retransmit.
+		if dbUpdate.Hint == update.Hint {
+			return session.TowerLastApplied, nil
+		}
+
+		// Otherwise, fail since the breach hint doesn't match.
+		return 0, wtdb.ErrUpdateAlreadyCommitted
+	}
+
+	// Sequence number must increment.
+	if seqNum != session.SeqNum+1 {
+		return 0, wtdb.ErrCommitUnorderedUpdate
+	}
+
+	// Save the update and increment the sequence number.
+	session.CommittedUpdates[seqNum] = update
+	session.SeqNum++
+
+	return session.TowerLastApplied, nil
+}
+
+// AckUpdate persists an acknowledgment for a given (session, seqnum) pair. This
+// removes the update from the set of committed updates, and validates the
+// lastApplied value returned from the tower.
+func (m *ClientDB) AckUpdate(id *wtdb.SessionID, seqNum, lastApplied uint16) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	// Fail if session doesn't exist.
+	session, ok := m.activeSessions[*id]
+	if !ok {
+		return wtdb.ErrClientSessionNotFound
+	}
+
+	// Retrieve the committed update, failing if none is found. We should
+	// only receive acks for state updates that we send.
+	update, ok := session.CommittedUpdates[seqNum]
+	if !ok {
+		return wtdb.ErrCommittedUpdateNotFound
+	}
+
+	// Ensure the returned last applied value does not exceed the highest
+	// allocated sequence number.
+	if lastApplied > session.SeqNum {
+		return wtdb.ErrUnallocatedLastApplied
+	}
+
+	// Ensure the last applied value isn't lower than a previous one sent by
+	// the tower.
+	if lastApplied < session.TowerLastApplied {
+		return wtdb.ErrLastAppliedReversion
+	}
+
+	// Finally, remove the committed update from disk and mark the update as
+	// acked. The tower last applied value is also recorded to send along
+	// with the next update.
+	delete(session.CommittedUpdates, seqNum)
+	session.AckedUpdates[seqNum] = update.BackupID
+	session.TowerLastApplied = lastApplied
+
+	return nil
+}
+
+// FetchChanPkScripts returns the set of sweep pkscripts known for all channels.
+// This allows the client to cache them in memory on startup.
+func (m *ClientDB) FetchChanPkScripts() (map[lnwire.ChannelID][]byte, error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	sweepPkScripts := make(map[lnwire.ChannelID][]byte)
+	for chanID, pkScript := range m.sweepPkScripts {
+		sweepPkScripts[chanID] = cloneBytes(pkScript)
+	}
+
+	return sweepPkScripts, nil
+}
+
+// AddChanPkScript sets a pkscript or sweeping funds from the channel or chanID.
+func (m *ClientDB) AddChanPkScript(chanID lnwire.ChannelID, pkScript []byte) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if _, ok := m.sweepPkScripts[chanID]; ok {
+		return fmt.Errorf("pkscript for %x already exists", pkScript)
+	}
+
+	m.sweepPkScripts[chanID] = cloneBytes(pkScript)
+
+	return nil
+}
+
+func cloneBytes(b []byte) []byte {
+	bb := make([]byte, len(b))
+	copy(bb, b)
+	return bb
+}