Chris Pratt, Katherine Schwarzenegger Are Instagram Official

Marvel star Chris Pratt and author Katherine Schwarzenegger have finally made their relationship Instagram official.

On Thursday, Dec. 13, 2018, the Guardians of the Galaxy star confirmed his long-rumored romance with the eldest daughter of Arnold Schwarzenegger and Maria Shriver with a sweet birthday message on the popular social media site.

"Happy Birthday Chief!" Pratt wrote, captioning a collage of cute snapshots of the happy couple. Completing the post with a heart and other celebratory emojis, he added, "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."

As Nicki Swift previously reported, rumors that Pratt, 39, and Schwarzenegger, 29, were dating first surfaced back in June 2018. After being spotted together on multiple occasions throughout the summer, including sharing a kiss during an afternoon outing in July and later enjoying a weekend getaway in Napa, Calif., speculation over their blossoming romance continued to grow. In late October, the then-alleged pairing were reportedly considering moving in together.

As far as we know, Pratt's long-suspected relationship with Schwarzenegger marks his first serious girlfriend since splitting from ex-wife Anna Faris. He and the 42-year-old Mom star, who share 6-year-old son Jack together, first announced their separation in April 2017 after eight years of marriage. While continuing to successfully co-parent, the amicable exes finalized their divorce this past October and will share joint custody of their only child. Meanwhile, Faris has been romantically linked with cinematographer Michael Barrett since October 2017. The Unqualified podcaster reportedly met her new man on the set of Overboard.

In other celeb relationship news this week, it was revealed that One Direction's Niall Horan and singer-actress Hailee Steinfeld reportedly called it quits earlier this fall. According to a source cited by E! News on Thursday, Dec. 13, "Hailee and Niall were going strong over the summer but split a few months ago and have been trying to keep it low-key." Calling the former couple's relationship "young love," the insider said that Horan and Steinfeld "really tried to make it work," but claimed that the latter's "insanely busy" schedule allegedly played a role in their ultimate breakup.