"It was just too soon to enter into a lifelong commitment," their representatives said in a joint statement. "It is better to have a brief engagement than a short marriage. The couple continue to share their time together and remain open to whatever the future may hold."
Stewart, the 26-year-old daughter of singer Rod Stewart, and Torriero, a 19-year-old star of the MTV reality show, announced their engagement earlier this month.
Torriero's publicist, Jack Ketsoyan, said early Sunday that the two were no longer romantically involved. Stewart's representative, Elliot Mintz, said they "remain friends."
Mintz said they began dating "a relatively short period of time" ago.
"It would be in the category of weeks as opposed to months," Mintz said.
Stewart and Torriero were recently in the news as the backseat passengers of Paris Hilton when she was involved in a minor car accident Nov. 9. No one was hurt and police said no one was cited after the mishap.
Stewart is a close friend of Hilton's and was at one time reported to be a possible replacement of Nicole Richie on the canceled Fox reality show "The Simple Life."
Torriero has said he's attempting to start a music career.