Chris Pratt has made his relationship with Katherine Schwarzenegger Instagram official.

Chris Pratt's birthday collage for Katherine (c) Instagram

Chris Pratt's birthday collage for Katherine (c) Instagram

The 39-year-old actor has been romancing the author, the daughter of Hollywood icon Arnold Schwarzenegger, since June, and things seem to be going from strength to strength for the pair as the Hollywood hunk shared a touching tribute to his girlfriend on the social media app on her 29th birthday (13.12.18).

Chris admitted that he has "cherished" being with the brunette beauty and is "thankful" for the "love and care" she brings into his life.

Alongside an adorable collage containing fun pictures of the pair, including one of Chris planting a kiss on Katherine's cheek, he wrote: "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. (sic)"

Despite the relationship getting "serious", sources recently claimed Chris isn't likely to be getting down on one knee any time soon.

An insider said: "[They are in a] serious relationship and everything is going well. However, Chris just finalised his divorce from Anna [Faris] a couple months ago, and he's happy with just dating Katherine for now.

"The couple has a lot of fun together and is fully involved in one another's life. [They are] in a committed relationship. There just isn't a rush to take it to the next step right now. Chris is enjoying the dating process."

The 'Guardians of the Galaxy' star - who has six-year-old son Jack with ex-wife Anna Faris - is even said to have won over Katherine's father Arnold, after the three met up with Arnold's girlfriend Heather for brunch over the weekend.

One source said: "Chris and Katherine met up with Arnold and his girlfriend Heather for Sunday breakfast at the FIG restaurant. Arnold seemed happy that Chris and Katherine joined him."

Whilst another insider added: "A lot of laughter and storytelling. Arnold seemed very pleased to spend time with Chris. It was obvious that they got along and enjoyed getting to know each other. They had eggs, breakfast meats and fruit. Chris settled the check."