"Happy birthday bunny!!!!!!" it read, alongside a photo of them both enjoying the sun.
He also posted an adorable shot of them together, which looks like it was taken on New Year's Eve.
"You're a dream," he wrote, nearly kissing her in the photo. He added a smile and love heart emoji.
They're so cute!
The pair were first linked in December 2019.
Photos obtained by E! News showed Ariel and Luke went on a date at Delilah in West Hollywood at the time, after she had gone and supported his reading of "Really Really" at the Lee Strasberg Theatre & Film Institute.
A source - who allegedly laid witness to the adorbs date - told E! News: "It was very clear they are a couple and they were affectionate all night long. They had their arms wrapped around each other and he put his hand on her waist.
"They took pictures together with his phone and posed with their faces together."
Another source told E! that the new couple were hanging out until the wee hours of the morning.
"They came out [of Delilah] around 1:45 a.m. and stood rubbing each other's backs as they waited for their car. They never took their hands off of each other."
Luke and Ariel even hit the red carpet together on December 5th for the Lancôme x Vogue L'Absolu Ruby Holiday Event, so it all seems pretty official to us!
