Multiple sources have confirmed the news following months of speculation about their seven-month relationship.
"Everyone thinks it's wild but are so happy for them," an insider told Us Weekly. "Benji always tends to fall in love easily, but this time it's for real and he landed a great girl. They obviously both make each other incredibly happy and there's nothing better than that."
In August a source told People magazine the couple had already talked about possibly getting married.
News of the alleged engagement comes two months after Diaz sparked speculation that she was engaged after stepping out with a diamond ring on her ring finger.