Chris Pratt Wishes Katherine Happy Birthday ... Reveals Pet Name

12/13/2018 6:22 PM PST

Chris Pratt Wishes Katherine Schwarzenegger Happy Birthday, Reveals Pet Name

Chris Pratt sending birthday love to his girlfriend, Katherine Schwarzenegger, was expected ... but he shocked us all by spilling the beans on his super cute nickname for her!!!

Chris posted an adorable bday message to Katherine on his Instagram, complete with a collage that screams #couplegoals ... but the thing that really caught our eye was the pet name, Chief. 

The social media equivalent of a birthday card reads, "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. 

It's interesting ... as best we can tell, this is the first time Chris has ever shared a pic of Katherine on his social media pages. They've been having tons of fun together since the summer, from family football games on Thanksgiving to PDA sessions at Disneyland ... and now it's social media-official.

Turns out, Chief isn't just a nickname for softball buddies ... celeb couples love it too!