"Sweet Katherine, so happy you said yes! I'm thrilled to be marrying you. Proud to live boldly in faith with you," he wrote alongside a sweet picture of the pair embracing.
Out promoting his new film, Lego Movie 2: The Second Part, ET asked about the actor's plans.
"The future? Oh. Lots of kids," Pratt said. "Maybe less time spent working, more time spent enjoying life. I always want to be that guy that works to live, not lives to work. I think [I want to spend] just a lot of time at the farm and a lot of time catching a lot of fish and seeing lots of sunsets."
Pratt already shares six-year-old son Jack with ex-wife Anna Faris.
Speaking specifically about his fiancé, Pratt would only say: "I'm a lucky man."
Schwarzenegger has stayed relatively hush-hush on the engagement, only re-posting Pratt's announcement photo to her own social media page, with the caption: "My sweet love. Wouldn't want to live this life with anyone but you."
Schwarzenegger's mum Maria Shriver, however, spoke about her daughter's current situation. "She's super happy and gave me super strict instructions not to talk about it," Shriver said. "It's not my moment. She wants to have her moment."
Watch below: Inside Newly Engaged Couple Chris Pratt and Katherine Schwarzenegger's Whirlwind Romance.