Santa Ana is a city in California. It is the county seat and has the most people of any city in Orange County. It is the 53rd biggest city in the United States. In 2007, 339,555 people lived in Santa Ana, and it is guessed that the city had 355,662 people living there in 2009. The city was founded in 1869. It is in Southern California on the Santa Ana River, away from the ocean. The 2000 United States Census said that Santa Ana has the 4th most dense population of US cities with more than 300,000 people.