On December 7, 2018, Rossi announced that he would be joining Acura Team Penske for both the 24 Hours of Daytona and the 12 Hours of Sebring in the 2019 IMSA WeatherTech SportsCar Championship. Rossi replaced fellow IndyCar driver Graham Rahal after Rahal was unable to get an adequate seat fitting to optimize his performance. Rossi teamed with fellow IndyCar Series driver Hélio Castroneves and sports car driver Ricky Taylor to finish third overall and in class at Daytona. The second half of the race was marred by non-stop rain, with Rossi conceding that the team's rivals were slightly stronger than them in wet conditions.
Rossi's 2018 season began with a controversial third-place finish at the Grand Prix of St. Petersburg. Rossi was running in second on the final restart and attempted to pass leader Robert Wickens in the first turn, but got loose and made contact with Wickens, who in turn retired from the race. Rossi argued that while he "[felt] bad" about the contact, he also believed Wickens' defense of the lead forced him into the marbles, causing his car to get loose. Wickens, however, said he "expected more" from Rossi, and proceeded to call his move "desperate." Rossi's driving was further criticized by NBC Sports' Jerry Bonkowski, who opined that he "ruined what was a near-perfect day — make that weekend — for another driver." Ironically, Rossi would again battle with Wickens in the closing stages of the following race at Phoenix, securing another third-place finish behind race winner Josef Newgarden and Wickens. After the podium celebration, both Wickens and Rossi made light of the St. Petersburg clash; Rossi stated that Wickens "should definitely have two podiums right now," to which the Canadian jokingly replied, "About time you said it!" The following week, Rossi qualified on pole position for the Grand Prix of Long Beach. He converted the pole position into his third career win, leading race runner-up and 2014 IndyCar Series champion Will Power to call Rossi "a standout of the field right now in every respect."
Rossi competed in his first ever Baja 1000 in 2018, driving with Jeff Proctor in the No. 709 Honda Ridgeline, becoming the fifth Indianapolis 500 winner to participate in the race. Rossi explained that he was eager to try a different racing series, saying, "It's just one of those iconic races. There's a Honda connection, so [when offered the ride] I just figured, 'Why not?' I'm pretty excited about it. It's something new, something different. I'm looking forward to the experience." Rossi nearly collided with a spectator's car that was travelling the wrong way on the course around 33 miles into the race. His team placed second in their class in the final results.
Rossi appeared on the 30th season of CBS' The Amazing Race with fellow IndyCar driver Conor Daly as his teammate. The pair won the fourth leg of the race, though they were eventually eliminated, finishing in fourth place. On April 19, 2018, he appeared on the season four premiere of CNBC's Jay Leno's Garage alongside Tanner Foust.