The actor shared a picture of the couple in an embrace, with a rock visible on Schwarzenegger's ring finger.
Excited about the news, Pratt, 39, wrote, "Sweet Katherine, so happy you said yes!"
"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".
The 39-year-old actor posted a photo of him and Katherine, 29, embracing each other on Instagram as she admired her enormous engagement ring. Proud to live boldly in faith with you. Schwarzenegger is the eldest daughter of Arnold Schwarzenegger and Maria Shriver.
Rumors of the couple dating began in June when they were snapped during a picnic in California, as per the Daily Mail.
The pair have one son together, six-year-old Jack.
In a post in December, Pratt marked Schwarzenegger's 29th birthday by sharing a picture collage of them together. Thrilled God put you in my life.
Pratt previously Wednesday Faris in 2009. Pratt finalized his divorce from ex-wife actress Anna Faris in October 2018.
- U.S. bus driver hailed for rescuing toddler
- Murray will be a big loss - Nadal pays tribute to retiring great
- LeBron James Getting Crushed For What He Did After Cowboys-Rams
- Gunman who fatally shot Davis Police Officer Natalie Corona is identified
- Shubman Gill credits Dravid for his batting evolution
- Stunning TVs From CES 2019 You'll Want in Your Living Room
- FBI opened investigation on Trump working on behalf of Russian Federation
- Wisconsin kidnapping and killing suspect lived under the radar
- Iran air force cargo plane crashes, killing at least 15
- Mass Fentanyl Overdose in California Kills One, Sends 12 to Hospital