Wearing a ring on her engagement finger at Sunday night's MTV Movie Awards, Reed responded to wedding inquiries by saying, "He's the one!"
Reed, 23, and McDonald, 26, began dating in March after they met at the Red Riding Hood premiere while McDonald was competing on this season of Idol.
An e-mail to Reed's rep was not immediately returned.
McDonald, who placed eighth on the singing show, told TVGuide.com after his elimination that he had never seen any of the Twilight saga, but planned to become a fan.
Watch McDonald gush about Reed: