The investigation into the death of Natalie Wood -- the huge star who died in 1981 in a drowning accident near Catalina Island in California -- has been re-opened as a homicide investigation ... and law enforcement sources tell us ... they are now privy to new information about her death.

Wood was boating with her husband, Robert Wagner and fellow actor Christopher Walken on Thanksgiving weekend 30 years ago when she went overboard.  There have been reams of stories about her death ... some suggesting foul play, but none substantiated.

Officials ruled her death was an accident, but now we're told new information has surfaced and the investigation has been re-opened.

The L.A. County Sheriff will hold a news conference Friday at 11 AM PT.  Of course, we will be livestreaming it.