Geary shared the happy news on Instagram, posting a photo and video of the moment.
Two black-and-white photos find the couple posing in front of a Christmas tree, with Thicke first kissing Geary on the cheek before the two exchange a kiss.
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In the clip, Thicke and Geary can be seen sitting at a busy dinner table, with Geary holding her hand over her mouth as Thicke holds out a ring box.
"Are you serious?" Geary asks the musician, who responds in the affirmative before the pair shares a kiss.
"YES YES 1000x YES," Geary captioned the slideshow along with two crying emojis and a ring emoji.
A ring wasn't the only thing Thicke surprised his fiancée with on Christmas Eve, as Geary revealed on Instagram that the 41-year-old had gifted her with a sweatshirt featuring a photo of their 10-month-old daughter Mia Love.
"BEST CHRISTMAS PRESENT, I LOVE MY MIA SWEATSHIRT BABE," Geary wrote alongside a photo of herself modeling the pink sweatshirt in front of a Christmas tree.
Geary is currently pregnant with the couple's second child together, and the baby is due in the spring. The pair originally announced Geary's pregnancy in August with a video and photos of baby Mia holding her sibling's sonogram images in her hands.
"Mia, what's that?" Geary asks her daughter in the clip. "What are you holding? Is that your little baby brother or sister? Are you excited?"
"Well someone is going to be a big sister next year!" the 23-year-old captioned the post. "We’re so excited to share with y’all that I’m expecting again! We find out Saturday if it’s a boy or a girl! What do you guys think it’s going to be?"
She added that her baby's due date falls on Thicke's birthday, which is March 10.
Soon after that post, the couple shared that they are expecting another daughter after attending a reveal party at CATCH restaurant in Los Angeles.
“Going to have another daddy’s girl!!! I love you baby!" Geary wrote on Instagram after the bash. "thank you @markbirnbaum for doing the best sex reveal ever!!!!! At my favorite restaurant @catch."0comments
In addition to Mia, Thicke is also dad to 8-year-old son Julian with ex-wife Paula Patton. Thicke and Geary have been dating for four years.
Photo Credit: Getty / Axelle/Bauer-Griffin