Purdy entered his true freshman year at Iowa State in 2018 as the third string to Kyle Kempt and Zeb Noland. He became the starter after Kempt was injured and Noland was ineffective. Overall he started eight games, completing 146 of 220 passes for 2,250 yards, 16 touchdowns and seven interceptions. He also rushed for 308 yards and five touchdowns. He returned as the starter in 2019.