Asghari, 25, took to Instagram to share a cute throwback video of the couple ice skating to Spears' hit "Break the Ice." Seconds into the clip, the actor, who appears a little shaky in his skates, totally wipes out. (Note that there's profanity in the video.)
"We live, we laugh, we argue, and most of all we LOVE. There’s a reason why the world fell in love with you! You might be the princess of pop but mostly you’re my princess," Asghari gushed in the caption.
"Biggest Happy Birthday to you @britneyspears," he added. "My A-- still hurts from falling and breaking the ice, you get it? #breaktheice."
Spears and Asghari have been dating since 2016. Though the couple mostly keep a low profile, they often share pics from their life together on Instagram.
Asghari opened up about the relationship to "Entertainment Tonight" in September, sharing that he "absolutely" plans to pop the question one day.
“This is something that every couple should do. That's the whole point of a relationship — we are a family," he said. He added that he has an "amazing" relationship with Spears' sons, Sean, 14, and Jayden, 13 (both with ex-husband Kevin Federline).
Asghari was hardly the only one to wish the "Toxic" singer a happy birthday. Spears' little sister, country singer Jamie Lynn Spears, got in on the action, too.
The "Zoey 101" alum shared a slideshow of pics of the close-knit sisters together. One adorable snap, taken when they were little girls, shows both sisters sporting bikinis and curly hair.
"When Britney got a perm, well I had to have one too," Jamie Lynn explained in her caption. "Some things never change. Always looking up to you. Happy Birthday to my sister."
Happy birthday, Britney!