"Avengers: Infinity War" will feature the return of Gwyneth Paltrow and Jon Favreau.
"Infinity Trinity ... #infinitywar and street cred @jimmy_rich represent @marvelstudios and @therealstanlee," he captioned a picture on Instagram.
Paltrow played Pepper in the "Iron Man" movies, "The Avengers" and "Spider-Man: Homecoming," while Favreau appeared in the "Iron Man" films and "Spider-Man: Homecoming."
Favreau also directed "Iron Man" and "Iron Man 2," and will produce "Avengers: Infinity War."
"#infinitywar Infinity Trinity," the 50-year-old actor and director captioned Downey's photo on his own account.
"Captain America: The Winter Soldier" and "Captain America: Civil War" helmers Anthony and Joe Russo will direct "Avengers: Infinity War."
The movie will see Tony Stark and the Avengers unite with the Guardians of the Galaxy to confront Thanos.
Josh Brolin will portray Thanos, with Mark Ruffalo as the Hulk, Chris Evans as Captain America, Chris Hemsworth as Thor, Scarlett Johansson as Black Widow, Tom Holland as Spider-Man and Chris Pratt as Star-Lord.
"This is a culmination film ten years in the making and we are pulling the main story threads from all of the different franchises into one unifying narrative," Joe Russo told Screen Rant in July.
"Avengers: Infinity War" opens in theaters in May 2018.
Robert Downey, Jr./Instagram