Alleged Gilgo Beach serial killer Rex Heuermann is expected to be linked to an additional victim when he pleads guilty tomorrow to a series of brutal killings on Long Island ... potentially running his body count to 8.

Heuermann is expected to plead guilty Wednesday to murdering Melissa Barthelemy, Maureen Brainard-Barnes, Amber Lynn Costello, Sandra Costilla, Valerie Mack, Jessica Taylor and Megan Waterman -- and he might cop to an 8th murder too ... according to a report Tuesday from NewsNation.

The identity of the 8th potential murder victim has not been released.

Heuermann was gearing up to go to trial in September on murder charges ... but he's now reportedly planning to cut a deal with prosecutors and plead guilty.

Law enforcement has so far found human remains of 11 different people between 1996 and 2011 near Gilgo Beach, New York ... but the reopened investigation didn't lead them to Heuermann until 2023, when he was arrested that summer.