IndyCar runner-up Alexander Rossi will replace Graham Rahal in Penske's Acura line-up for the 2019 IMSA SportsCar Championship's endurance rounds.
Rossi, the 2016 Indianapolis 500 winner, will join full-timers Helio Castroneves and Ricky Taylor in the #7 Acura ARX-05.
That car led a Penske one-two at Mid-Ohio last season, Penske's first full sportscar campaign since its American Le Mans Series Porsche programme ended in 2009.
Rahal said on social media that he had struggled to fit into the Acura.
Rossi's fellow IndyCar driver Simon Pagenaud will continue for a second year as the endurance pilot for the #6 Acura, alongside Juan Pablo Montoya and Dane Cameron.
"I have watched and raced against Team Penske in IndyCar and I have a lot of respect for the organisation and what they do and how they do it," said Rossi, who races for Andretti Autosport in IndyCar.
"I really appreciate Andretti Autosport allowing me the opportunity to represent Acura in some of the biggest sportscar races in the world.
"I can't wait to work with Helio and Ricky, to learn from them and hopefully contribute to some wins and a championship for Acura Team Penske."
Honda Performance Development president Art St Cyr described Rossi as "a great addition" to the Acura programme.
"When you add Alexander to the experienced team of Helio, Ricky, Juan and Dane - and with Simon returning for the endurance events - we are extremely proud of the talented and accomplished Acura Team Penske driver line-up," said St Cyr.
"We look forward to our second opportunity to win the Rolex 24 at Daytona in the ARX-05.
"Alexander has been a great representative of Honda in the IndyCar Series, from his Indy 500 victory in 2016 to competing for the title this past season.
"We are pleased to have this opportunity to build our relationship with him through our sportscar programme."