The singer, who officially turned 27 years old Tuesday, kicked off her birthday celebration Monday with Grande and her manager, Scooter Braun, in London.
Lovato enjoyed birthday cake with Grande, Braun and other friends after attending Grande's concert at The O2 Arena Monday evening. Lovato shared a video of herself blowing out the candles on her cake.
"They literally filmed/sang happy birthday twice and @scooterbraun STILL forgot to press record," she captioned the post. "@arianagrande's face is everything and this video is a perfect glimpse of how [expletive] happy and rad my birthday is this year. So so happy. And I love my new family. Thank you guys, love you."
Earlier in the evening, Lovato shared a sweet moment with Grande backstage prior to the singer's show.
"This was too sweet not to post.... before show prayer they did this for my bday," Lovato wrote. "I'm so so proud of you @arianagrande ... I love you tons."
Grande responded to Lovato in the comments.
"love us so much!!! thank u for coming!!!!" she wrote.
"Can't love anyone unless you love yourself first," she said on Instagram Stories.
Lovato said in June she will tell her "side of the story" on her new album. Lovato last released the single "Sober" in June 2018, shortly before her overdose.
"You know what's great about making an album? You get to say anything you want, be as open and honest as possible and finally tell your side of the story regardless of who might not like it," Lovato said on Instagram Stories.