Vallejo (/vəˈleɪ.oʊ/ or /vəˈleɪhoʊ/Spanish[baˈʎexo]) is the largest city in Solano CountyCaliforniaUnited States. The population was 115,942 at the 2010 census. It is the tenth most populous city in the San Francisco Bay Area, and is located on the northeastern shore of San Pablo Bay. Vallejo is named for General Mariano Guadalupe Vallejo and the neighboring city of Benicia is named for his wife.

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. The colleges and universities in Vallejo are California Maritime Academy, the Vallejo Center campus of Solano Community College, and Touro University CaliforniaFerryservice runs from a terminal on Mare Island Strait to San Francisco, through the BayLink division of SolTrans.

Vallejo has twice served as the capital of the state of California: once in 1852 and again in 1853, both periods being brief.[9] The State Capitol building burned to the ground in about 1858. The only surviving picture of the capitol was taken about 1858 and appears to show hay being stored on the second floor. and the Vallejo Fire Department requested aid from the Fire Department at Mare Island Naval Shipyard. As there were no bridges at that time, the Mare Island Fire Department had to be ferried across the Napa River, arriving to find only the foundation remaining. This was the first recorded mutual aid response in the state of California.

Some of the first Europeans drawn to the Vallejo area were attracted by the sulfur springs; in the year 1902 the area was named Blue Rock Springs. It was also known as White Sulfur Springs and that was the name of the road leading to it from town, today shortened to "Springs Road."