Does a US citizen need a visa to leave Istanbul airport?

YesYou need a visa to leave the airport.

Do I need a transit visa for Turkey for a stopover? You do not need a visa if the transit takes only 24 hours, but if you stay at the airport all the time.

If you want to leave, you must get a visa.

The system for applying for an electronic visa (e-Visa) was introduced on 17 April 2013 by the Ministry of Foreign Affairs of the Republic of Turkey. With this system, visitors traveling to Turkey can conveniently get their electronic visa online in about three minutes. The e-visa costs 20 USD,