The "Sorry Not Sorry" star, who actually turns 27 on Tuesday, showed her support for her famous friend as she popped along to her gig at the O2 Arena alongside manager Scooter Braun.
Demi took to Instagram to post a short clip of a birthday message Ariana and her dancers had shared for her prior to hitting the stage and wrote: "This was too sweet not to post.... before show prayer they did this for my bday. I'm so so proud of you @arianagrande. You f**king killed that!!! I love you tons."
Replying to the post, Ariana wrote: "love u so much !!! thank u for coming !!!".
Demi also lauded the 26-year-old star with numerous posts on her Instagram Stories, many of which praised Ariana as a "queen". Other clips showed her rocking out in the audience alongside Scooter, who also looks after Ariana, and a group of friends.
The "Cool for the Summer" singer's friendship and working relationship with Scooter hit headlines earlier this year when she sided with the music manager in his feud with Taylor Swift.
The "ME!" hitmaker was stunned when Scooter took control of her former label Big Machine - and with it the masters of her first six albums - and she made it clear she wasn't a fan of him in an online blast.
Demi spoke out in support of Scooter, but was met with such backlash from Taylor fans that she decided to take a break from social media "for a while".