The 24-year-old actor and comedian dedicated sweet posts to Grande on Instagram before joining the singer at her early birthday party Monday.
Davidson shared a throwback photo of himself giving Grande a piggyback ride backstage. He called Grande his "favorite person" in the caption.
"happy birthday to the most precious angel on earth! you're my favorite person that ever existed :) i love you sm," the actor wrote.
Davidson followed up with an Instax photo of himself with Grande.
"One more for the queen," he captioned the post.
Grande spent Monday evening with Davidson, brother Frankie Grande and other family and friends at the Mandarin Oriental hotel in New York, according to E! News. Frankie wished his sister a happy birthday in a post Tuesday.
"i remember seeing her for the first time and promising that i would love her every day fr the rest of my life. it has been 25 years to the day and that promise has held true," the 35-year-old television personality wrote.
"Ariana, you are my best friend, biggest supporter, and greatest little sister a big brother could ever ask for. thank you for being you & happy happy birthday. can't wait for the next 25 years and many many more..." he added.