Heard, 27, was spotted this week wearing a large diamond on her ring finger.
"The proposal happened a while ago," one insider told the magazine. "She just didn't wear the ring."
Depp, 50, split from his longtime partner, Vanessa Paradis, in 2012. They have a daughter Lily-Rose, 14, and son Jack, 11.
Celebrity Couples of 2014 [PHOTOS]