Showrunner, Dan Fogelman announced the news on Thursday at the Television Critics Association summer press tour, according to TV Line.
The Oscar-nominated actor will be portraying the co-star and father figure in the Ron Howard movie that Justin Hartley's character, Kevin Pearson was cast in at the end of season one.
Fogelman says that Milo Ventimiglia, who plays Jack Pearson, was the one who got the ball rolling with Stallone. Ventimiglia, 40, played Stallone's son in the sixth Rocky movie, Rocky Balboa.
"Isn't that amazing?" Justin Hartley said. "We have these big ideas on the show, and you hear about it through the grapevine and think, 'Wow, that's massive. How are we going to pull that off?' Like, 'I'm not sure he does television…,' but man, I guess the show is that good that people are wanting to be a part of it at every level."
Hartley says that he and Stallone's characters are teaming up in a "war movie/period piece...a massive-budget, huge movie." He continued by saying, "We got the main man to come do it. It doesn't really get better than that, does it?"
This won't be the first time that Hartley has interacted with Stallone.
"We work out at the same gym sometimes, so I've met him [before]," Hartley said.
As for the second season, Fogelman says that there are a number of challenges that the showrunners are encountering.
"It's very heavy," he said while speaking with The Wrap. "Our big challenge is also keeping it fun. Obviously, there's a potential death at the center of the season, which naturally makes things sad."
Fogelman also addressed the highly emotional feel of the series.
"I think part of the thing people are talking about [with] the emotion of the show, it's not always sad emotion. It's an uplifting emotion — it's a cry that makes you smile. We want to make sure the show also remains fun and funny."
This Is Us returns on Sept. 26 on NBC at 9 p.m. EST. Check your local listings.
Photo Credit: Trae Patton