The post was accompanied by a Snapchat video of the two jamming out in the car. Naturally, Grande used one of the app's popular face-altering filters.
"Happy birthday to my best friend for life / soulmate @larryfisherman," Grande, 23, captioned it. "I love you with all my heart."
The former Nickelodeon star and Miller have been friends since the two collaborated on her catchy 2013 track, "The Way," but didn't start dating until they reconnected last September. Since then, the pair hasn't been shy about expressing their love for each other via social media or packing on the PDA while out and about.