He formally joined Penske's 2018 IMSA program in October 2017. Castroneves was joined by Ricky Taylor for the entire season, and by Graham Rahal for the 24 Hours of Daytona, the 12 Hours of Sebring and Petit Le Mans. He referenced his teammates Dane Cameron and Montoya, and communicated constantly with his race engineer and Taylor. Their car started second for the Daytona race, and finished ninth, after Castroneves and Action Express Racing's Felipe Nasr collided in the 16th hour. After starting third at Sebring, he retired after six hours with a loss of oil pressure. In the Acura Sports Car Challenge at Mid-Ohio he and Taylor led 87 laps from pole for his first series victory. The final six races saw him achieve four top tens and finish second in the Chevrolet Sports Car Classic at Detroit. Castroneves was seventh in the Prototype drivers' standings with 243 points and was third in the North American Endurance Cup.