The "Iron Man" actor took to Instagram early on Wednesday, May 01 to share two photos from the behind-the-scenes gathering, for which he invited his co-stars Gwyneth Paltrow, Zoe Saldana, Brie Larson, Elizabeth Olsen, Karen Gillan, Letitia Wright, Evangeline Lily, Danai Gurira, and Pom Klementieff to join him in a make-shift tent outside his trailer for a celebratory meal.
"#throwback#flashback to the #Women of @marvelstudios #mcu (Marvel Cinematic Universe) lunch I had the pleasure of hosting...," he captioned the shots, adding the hashtags, "#girlpower" and "#epic".
Robert didn't explain the reason for the bash, which some of his co-stars attended while still in costume, but the all-female lunch line-up appeared to be in tribute to a moment of female empowerment featured in the final "Avengers" movie.
Scottish actress Karen, who plays Nebula in the comic book franchise, was quick to comment on the post, writing, "Yes!!!!!! PS thanks for lunch".
The cast and crew have been given plenty of reason to celebrate as "Avengers: Endgame" has continued to shatter box office records worldwide since its launch last week (ends April 26).
It opened with a stunning $1.2 billion global weekend take, and is currently on course to become the biggest film of all time and dethrone 2009 fantasy epic "Avatar".
Paltrow, who portrays Pepper Potts, marked the monster debut on social media on Sunday, when she uploaded a picture of herself with Downey Jr., Don Cheadle, and Scarlett Johansson on the set of 2010's "Iron Man 2".
"What a weekend! From Ironman 2 with this crew all the way to behind the scenes of #avengersendgame ...amazing to be a part of such an impactful franchise," she wrote.
"Congratulations to the giant team that made this film ("Endgame") possible, starting with ("Iron Man" director) @jonfavreau it all started with you."