Jason Wahler and Katja Decker-Sadowski
Wahler, 20, proposed to the 19-year-old USC freshman Saturday in front of a group of friends – including his ex-girlfriend Lauren Conrad – during a housewarming party in their Los Angeles home.
Wahler gave her a four-carat emerald cut diamond from XIV Karat in Beverly Hills.
No wedding date has been set for the couple, who recently moved in together.
Wahler, who is filming a new MTV show Celebrity Rap Superstar, returns to The Hills on the Sept. 10 episode.
After several clashes with the law – four drinking-related arrests on charges ranging from disorderly conduct to assault – Wahler recently completed two months in rehab for alcohol abuse.
"I relate to Lindsay [Lohan]," Wahler told PEOPLE in June, "because she's the same age as I am. I'm only 20. And to say that I can't drink ever again – even on my 21st birthday – is hard. But I don't care, as long as I'm healthy and happy."
• Reporting by MARK DAGOSTINO
