"Happy Birthday Chief! Your smile lights up the room. I've cherished our time together. Thrilled God put you in my life. Thankful for the laughs, kisses, talks, hikes, love and care," Pratt wrote.
The collage included a photo of Pratt kissing Schwarzenegger and another of the couple wearing animal printed sheet masks. One of the funny photos shows Schwarzengger painting Pratt's toes.
Pratt's fans rushed to the comments section to celebrate the couple.
"You make a good couple," one person wrote.
"So happy for you but a little jelly," another added.
"Aw... cute couple! I hope you're both happy together," another said.
Pratt, 39, and Schwarzenegger, 29, were first seen together in June and she is reportedly the first woman Pratt has dated since his marriage to Anna Faris ended. They were reportedly set up by Schwarzenegger's mother, Maria Shriver. They have been spotted by paparazzi photographers together, and the Round Pound Estate Winery shared photos of the couple together when they visited.
Sources have told media outlets that Pratt and Schwarzenegger are on the "fast track."
"Chris has fallen in love with Katherine and has told her that she is the last person he will ever kiss," a source told Hollywood Life in November. "They are without a doubt in the honeymoon phase of their relationship."
"She encompasses everything he wants in a partner. She's beautiful, smart and caring and understands his humor," the source continued, adding that Schwarzenegger is "wonderful and genuine" with Pratt's 6-year-old son with Faris, Jack.
Although Pratt and Faris' divorce was finalized in October, the two stars remain close friends. They even spent Halloween together with Schwarzenegger and Faris' boyfriend, Michael Barrett.
"They're such a great fit together. It's only been a few months but Chris and Katherine are both very serious about the relationship and their friends wouldn't be surprised if they got engaged sooner rather than later. Katherine has spent time with Chris' family and they all love her too. She fits in really well with them and vice versa," a source told PEOPLE in September.
Earlier this week, Pratt joined the cast of Disney and Pixar's Onward, along with Julia Louis-Dreyfus, Tom Holland and Octavia Spencer. The film will not release until March 6, 2020.0comments
Pratt's next movie is The LEGO Movie 2: The Second Part, which opens on Feb. 8, 2019. He also finished filming the Western The Kid with Ethan Hawke and Vincent D'Onofrio.
Photo credit: Getty Images