"Sweet Katherine, so happy you said yes! I’m thrilled to be marrying you. Proud to live boldly in faith with you. Here we go!" he wrote next to a picture of him kissing Katherine Schwarzenegger.
He added ring and heart emojis to the happy post.
The duo has been together since last summer -- and they have not hid their whirlwind romance from their fans and family.
Just this past December, Pratt posted a collage of pics of his love, writing, "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."
The happy news comes about a year and a half since Pratt split from Anna Faris.
"Anna and I are sad to announce we are legally separating," the pair wrote in the summer of 2017. "We tried hard for a long time, and we’re really disappointed. Our son has two parents who love him very much and for his sake we want to keep this situation as private as possible moving forward. We still have love for each other, will always cherish our time together and continue to have the deepest respect for one another."
The duo has continued to co-parent their 6-year-old son, Jack, and Schwarzenegger has also been seen spending time with the little guy.