TMZ has learned that the man who was stabbed at the home of celebrity fitness guru Gunnar Peterson has pleaded not guilty to felony assault charges stemming from the violent incident last December.

23-year-old Nick Marino, who is free on $300,000 bail, entered the plea at Beverly Hills Courthouse earlier today. Marino was stabbed by a security guard with a pocket knife after allegedly breaking into Peterson's Beverly Hills mansion.

Sources tell TMZ that Marino and Gunnar's estranged wife, Janet Crown Peterson, were at one point romantically involved, but the relationship ended roughly five months ago. Janet then alleged in court documents that Marino stalked her and threatened violence.

Neither Gunnar nor Janet were home at the time of the stabbing. If convicted, Marino could face up to four years in jail. He is due back in court March 8.