The actress, 32, posted photos of her being squeezed with the 47-year-old actress in honor of her birthday on her confirmed Instagram account.
"Thank you all for your birthday wishes and love! Cheers to another great year," she wrote in the caption in a series of photos.
There have been speculations for weeks that they were a couple, boosted by Affleck who wrote "Photo credit pls", in the comments section of a set of photos the actress posted in March and showed her on the beach.
The pair star together in the upcoming movie "Deep Water."
Affleck was married to actress Jennifer Garner from 2005 to 2018, and they are the parents of Violet, 14, Seraphina, 11, and Samuel, 8.