Surprise is a city in Maricopa County, in the U.S. state of Arizona. The population was 143,148 at the 2020 census, up from 117,517 in 2010 and just 30,848 in 2000. The city has a 10,562-square-foot Aquatics Center and Maricopa County's northwest regional library, a $5.5 million, 20,000-square-foot library, along with a 100.3 cost of living index.