Vallejo, California is in the United States.
Solano County is a county located in the U.S. state of California . Vallejo ( [ vəˈleɪ.oʊ , _ - hoʊ ] ; Spanish : [ baˈʎexo ] ) is a waterfront port city in Solano County , California , located in the North Bay region of the San Francisco Bay Area . It is the tenth most populous city in the San Francisco Bay Area , and the largest in Solano County . Vallejo is home to the Six Flags Discovery Kingdom theme park , the now-defunct Mare Island Naval Shipyard , and the regional office for Region 5 of the United States Forest Service .