Phoenix is the capital and most populous city of the U.S. state of Arizona, with 1,608,139 residents as of 2020. It is the fifth most populous city in the United States, the most populous state capital in the co…Phoenix is the capital and most populous city of the U.S. state of Arizona, with 1,608,139 residents as of 2020. It is the fifth most populous city in the United States, the most populous state capital in the country, and the only U.S. state capital with a population of more than one million residents.