Alameda Island is an island in the San Francisco Bay in California. It is south and west of, and adjacent to Oakland, and across the bay eastward from San Francisco. Located on the island is most of the city of Alameda, the island was originally a peninsula and a part of Oakland, and is now separated from the mainland by the Oakland Estuary. The island is connected to the mainland by four bridges: the Park Street Bridge, Fruitvale Bridge, High Street Bridge, and Bay Farm Island Bridge. The Posey and Webster Street tubes also connect Oakland to Alameda Island.