The Marvel star took to social media to wish her a happy 30th birthday and gush over what an "incredible" partner and step-mom she is.
"Happy Birthday Katherine! So happy to have you in my life," he wrote on Friday.
"I don’t know what I’d do without you. Probably get locked out on the balcony somehow and have to live there or be wandering around some city with an uncharged phone and only one shoe, late for work like some kind of anxiety fever dream.
"I honestly don’t even want to think about it. You’ve changed my whole world for the better. I’m so grateful to have found you.
'You’re an incredible wife and step mom. And I’m so excited to celebrate you all weekend!," the former "Parks and Recreation" concluded.
Pratt shares his seven-year-old son, Jack, with ex-wife Anna Faris. They were married from 2009 - 2018. He then tied the knot with Schwarzenegger in June in Montecito, California.
Meanwhile, Faris started dating cinematographer Michael Barrett in 2017.
In a previous interview with Fox News, the daughter of Arnold Schwarzenegger called their relationship “amazing."
“I think anyone who finds the person that they get to spend the rest of their life with… it’s a huge gift and a huge blessing, so I feel really lucky," she added.