Smith-Petersen announced the news Thursday on Facebook with a picture of himself and Keough with a large diamond ring on her finger. "So that happened," he captioned the photo.
Keough is a 25-year-old actress and model, and daughter to Lisa Marie Presley and Danny Keough. She is the eldest grandchild of Elvis and Priscilla Presley, and the first of the King's grandchildren to get engaged. Smith-Petersen is a stuntman, and the couple met while filming Mad Max: Fury Road in 2012.
"They're very serious," a source tells Us Weekly. "They're perfect for each other. Great people."
"So happy for them," a different insider adds. "They joked about having a dog 'baby' together now and how it was just a matter of time until they made it official."
Keough will appear as Capable in Mad Max: Fury Road, and is scheduled to star as Elisa Green in The Greens are Gone in 2015.