Parks and Recreations star Pratt held his bride-to-be close as they posed for pictures on the purple carpet in Los Angeles, four months after surprising fans with their engagement.
The pair looked as loved-up as ever with Pratt in a black suit and Schwarzenegger in a silver one-shoulder split dress.
Monday’s premiere marked the first time the couple have worked a carpet together, choosing the hotly-anticipated superhero film to mark the special milestone.
“Sweet Katherine, so happy you said yes! I’m thrilled to be marrying you,” he wrote. “Proud to live boldly in faith with you. Here we go!
The accompanying picture showed the pair in an emotional embrace, with Schwarzenegger’s ring on show. The news came a month after they had publicly confirmed their romance.
Newlyweds Miley Cyrus and Liam Hemsworth also dazzled photographers, with Cyrus showing off her figure in a strapless black dress, complete with a cutout around the midriff, while Liam kept it simple with a suit.
Pratt, who returns to the Marvel universe as Star-Lord in the upcoming film, and Schwarzenegger were among the famous faces at the star-studded premiere at the Los Angeles Convention Centre.
His brother Chris, who plays Thor in the saga, worked a blue patterned suit as he was supported by wife Elsa Pataky.
Brie Larson, Scarlett Johansson, Robert Downey Jr and Bradley Cooper were among those lining the purple carpet on Monday night ahead of the film’s release later this week. Gwyneth Paltrow, a.k.a Pepper Potts, was also pictured sharing a sweet embrace with her son Moses in a rare public appearance with her children.
The highly-anticipated sequel to Infinity War, released last year, is the culmination of 22 Marvel Cinematic Universe films, spanning over 11 years.
Plot details for Endgame – which picks up directly after Thanos wiped out half the universe – have been kept tightly under wraps, but actor Mark Ruffalo, who plays Hulk, has hinted that fans will be left “grieving”.
“I haven't seen the movie yet so I'm speaking out of turn but I think people are going to be grieving. And they're going to be hopeful,” he said at the premiere.
However, he added that fans will be “satisfied” with the mammoth film – which is three hours long – and its ending.
Avengers: Endgame is in cinemas from April 25