Law enforcement sources confirm Stan's wife, Kristyn Green, found the actor hanging Saturday afternoon in their bathroom. Paramedics responded, but we're told he was pronounced dead at the scene.

Stan had tons of acting credits, appearing on shows like "JAG," "General Hospital" and "Friends" ... in addition to his 6 seasons on "Highlander" in the mid-90s. However, he was also a renowned acting coach in Los Angeles.

A tribute to Stan on the show's official Facebook page said, "Without Stan Kirsch, Highlander: The Series would have been far less. He brought a sense of humor, kindness and youthful enthusiasm to the character of Richie Ryan for six seasons."

Stan's episode on "Friends" was a memorable cameo as a guy dating Monica ... with a twist that probably wouldn't play as well, or as funny, these days.

Stan later found success as Lieutenant Ferrari on "JAG" ... appearing for 5 seasons.

He attended Duke University as a political science major but he had already dipped his toe in the acting world by then. He was 4 years old when he landed his first gig ... in a Campbell's Soup commercial.

Stan's wife posted about his death, saying ... "I want to thank everyone for the outpouring of love and support. I haven't been able to respond to all the texts, calls, emails -- but have read or listened to every single one of them. I feel surrounded by love and am forever grateful to each and every one of you."

Stan was 51.