diff --git a/x/blockdb/memdb/database.go b/x/blockdb/memdb/database.go
new file mode 100644
index 000000000000..3f4ade00d8e7
--- /dev/null
+++ b/x/blockdb/memdb/database.go
@@ -0,0 +1,83 @@
+// Copyright (C) 2019-2025, Ava Labs, Inc. All rights reserved.
+// See the file LICENSE for licensing terms.
+
+package memdb
+
+import (
+	"sync"
+
+	"github.com/ava-labs/avalanchego/x/blockdb"
+)
+
+// Database is an in-memory implementation of BlockDB
+type Database struct {
+	mu     sync.RWMutex
+	blocks map[blockdb.BlockHeight]blockdb.BlockData
+	closed bool
+}
+
+// WriteBlock stores a block in memory
+func (m *Database) WriteBlock(height blockdb.BlockHeight, block blockdb.BlockData) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if m.closed {
+		return blockdb.ErrDatabaseClosed
+	}
+
+	if m.blocks == nil {
+		m.blocks = make(map[blockdb.BlockHeight]blockdb.BlockData)
+	}
+
+	if len(block) == 0 {
+		return blockdb.ErrBlockEmpty
+	}
+
+	blockCopy := make([]byte, len(block))
+	copy(blockCopy, block)
+	m.blocks[height] = blockCopy
+
+	return nil
+}
+
+// ReadBlock retrieves the full block data for the given height
+func (m *Database) ReadBlock(height blockdb.BlockHeight) (blockdb.BlockData, error) {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+
+	if m.closed {
+		return nil, blockdb.ErrDatabaseClosed
+	}
+
+	block, ok := m.blocks[height]
+	if !ok {
+		return nil, blockdb.ErrBlockNotFound
+	}
+
+	blockCopy := make([]byte, len(block))
+	copy(blockCopy, block)
+	return blockCopy, nil
+}
+
+// HasBlock checks if a block exists at the given height
+func (m *Database) HasBlock(height blockdb.BlockHeight) (bool, error) {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+
+	if m.closed {
+		return false, blockdb.ErrDatabaseClosed
+	}
+
+	_, ok := m.blocks[height]
+	return ok, nil
+}
+
+// Close closes the in-memory database
+func (m *Database) Close() error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	m.closed = true
+	m.blocks = nil
+	return nil
+}
diff --git a/x/blockdb/memdb/database_test.go b/x/blockdb/memdb/database_test.go
new file mode 100644
index 000000000000..1f101d4c123b
--- /dev/null
+++ b/x/blockdb/memdb/database_test.go
@@ -0,0 +1,77 @@
+// Copyright (C) 2019-2025, Ava Labs, Inc. All rights reserved.
+// See the file LICENSE for licensing terms.
+
+package memdb
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/ava-labs/avalanchego/x/blockdb"
+)
+
+func TestOperationsAfterCloseReturnError(t *testing.T) {
+	db := &Database{}
+
+	// Close database
+	require.NoError(t, db.Close())
+
+	height := blockdb.BlockHeight(1)
+	blockData := blockdb.BlockData("test block data")
+
+	// All operations should fail after close
+	err := db.WriteBlock(height, blockData)
+	require.ErrorIs(t, err, blockdb.ErrDatabaseClosed)
+
+	_, err = db.ReadBlock(height)
+	require.ErrorIs(t, err, blockdb.ErrDatabaseClosed)
+
+	_, err = db.HasBlock(height)
+	require.ErrorIs(t, err, blockdb.ErrDatabaseClosed)
+}
+
+func TestWriteReadAndHasBlock(t *testing.T) {
+	db := &Database{}
+
+	height := blockdb.BlockHeight(1)
+	blockData := blockdb.BlockData("test block data")
+
+	// Write block
+	require.NoError(t, db.WriteBlock(height, blockData))
+
+	// Verify HasBlock returns true
+	exists, err := db.HasBlock(height)
+	require.NoError(t, err)
+	require.True(t, exists)
+
+	// Read block back
+	retrievedBlock, err := db.ReadBlock(height)
+	require.NoError(t, err)
+	require.Equal(t, blockData, retrievedBlock)
+
+	// Verify non-existent block
+	nonExistentHeight := blockdb.BlockHeight(999)
+	exists, err = db.HasBlock(nonExistentHeight)
+	require.NoError(t, err)
+	require.False(t, exists)
+}
+
+func TestOverwritingBlockUpdatesData(t *testing.T) {
+	db := &Database{}
+
+	height := blockdb.BlockHeight(1)
+	originalData := blockdb.BlockData("original data")
+	updatedData := blockdb.BlockData("updated data")
+
+	// Write original block
+	require.NoError(t, db.WriteBlock(height, originalData))
+
+	// Overwrite with new data
+	require.NoError(t, db.WriteBlock(height, updatedData))
+
+	// Verify updated data
+	retrievedBlock, err := db.ReadBlock(height)
+	require.NoError(t, err)
+	require.Equal(t, updatedData, retrievedBlock)
+}