The actress and singer initially played coy about the news, posting a photo of her and her sister A.J. Michalka on Instagram. The photo is a full-bodied shot of both women, with Aly debuting her new engagement ring.
"Hmmm? no one's noticed yet. #lookcloser," she wrote in the caption accompanying the photo.
She later took to Twitter to explicitly announce the news.
Aly Michalka, 25, formerly appeared on Disney's Phil of the Future and acted alongside Emma Stone in Easy A.