Montoya, 38, will drive Penske's No. 12 Ford at Michigan International Speedway on June 15 and at the Brickyard 400 at Indianapolis Motor Speedway on July 27. The Verizon IndyCar Series is idle both weekends.
The Colombian is a former winner of the Indianapolis 500 who moved to Formula One in 2001, winning seven times over six years. Then he migrated to NASCAR stock car racing.
Montoya, driving for Chip Ganassi's NASCAR team, won two Cup races on curvy road courses but never won on an oval track in his 253 total Cup starts.
After being released by Ganassi after last season, Penske hired Montoya to return to IndyCar this year.
"Michigan and Indianapolis are two important [NASCAR] races for Team Penske," team owner Roger Penske said in a statement. "We know Juan has the ability to race for wins in both of those events while still focusing on his full-time program in the Verizon IndyCar Series."
"It will be fun to get back behind the wheel of a Cup car," Montoya said in a statement.
Although Penske has won a record 15 Indy 500s, Montoya noted that Penske has never won the Brickyard 400.
"I think Brad, Joey and I will give Roger and Team Penske a great opportunity to check that off the list, and we should be strong at Michigan as well," Montoya said.