The Nittany Lions (4-1) will play Nevada (2-2) on Saturday for the tournament championship.
The Cyclones (3-1) were within two points with 4:30 left in the game after a Jessica Schroll jumper, but the Nittany Lions closed on a 9-4 run.
Lucas matched her season high to lead three Penn State players in double figures. Zhaque Gray scored 14 points and Alex Bentley had 13.
Brynn Williamson led Iowa State with 15 points.
Penn State was up 37-28 at halftime and led by as many as 12 in the game. The Nittany Lions overcame 58 percent free throw shooting (15-for-26) and 32 fouls called against them compared to 18 by Iowa State.