Marilyn Mallon, 82, was in her Honda when it collided with a GMC about 10:15 a.m. at the LIE service road and County Road 24, police said.
Mallon had taken eastbound Exit 71 just before being hit by the GMC going south on CR 24, police said.
She was taken to Peconic Bay Medical Center in Riverhead, where she died from her injuries, police said.
The GMC driver, Tyler Yaskanich, 18, of Shoreham, suffered minor injuries, police said.
Both vehicles were impounded for safety checks.
Seventh Precinct detectives ask anyone with information to call 631-852-8752.