"Here we go", he captioned a picture that showed him kissing her. Katherine's large diamond engagement ring can also be seen in the picture.
The news came after Pratt went on a double date with Schwarzenegger's dad, Arnold, and his girlfriend Heather Milligan.
'Sweet Katherine, so happy you said yes!
Winter Weather Advisory In Effect Until Sunday
While Scotland will approach freezing temperatures on Monday afternoon most of England and Wales will see figures approaching 9C. So, if you're going down to watch the Eagles-Saints game at a Philly restaurant or with friends there, you should be fine.
The 39-year-old Guardians of the Galaxy actor broke the news on his Instagram account, sharing a picture of the couple snuggled up together with a big shiny rock on Katherine's fourth finger. Proud to live boldly in faith with you.
The "Avengers" star began dating Schwarzenegger, 29, earlier previous year. They continue to co-parent their six-year-old son, Jack. This was Pratt's first Father's Day since his divorce from Anna Faris. She is also the grandniece of John F. Kennedy; the mum of her mum, journalist Maria Shriver, is the former USA president's little sister.
They have also met each other's families, and it is believed that Schwarzenegger's mother, Maria Shriver, played cupid to get them together.