Ja'Net DuBois -- one of the main characters on "Good Times" -- died Monday ... TMZ has learned.

Ja'Net played the Evans family's sassy neighbor Willona Woods on the classic '70s TV series. Her family tells us ... Ja'Net died unexpectedly in her sleep overnight at her home in Glendale, CA.

We're told Ja'Net hadn't complained of any specific ailments recently, and appeared to be in good health to those around her. She even attended a fan event in Hollywood a couple weeks ago.

Ja'Net might be most famous for "Good Times," but she also composed and sang the theme song, "Movin' On Up," for "The Jeffersons" -- so, she should also be remembered for her voice.

She appeared in tons of movies over the years, including "I'm Gonna Git You Sucka", "Tropic Thunder" and "Charlie's Angels: Full Throttle."

Ja'Net's acting career began on Broadway, where she was cast in a number of plays including "Golden Boy" with Sammy Davis Jr. and Louis Gossett Jr. -- plus "A Raisin In The Sun."

She also won two Emmy awards for her voice-over work on "The PJs."

Ja'Net is survived by her 3 children.