Lindsay Lohan was in a bad car accident and is in the emergency room at an L.A. hospital ... TMZ has learned.

Lindsay Lohan was driving her black Porsche on the Pacific Coast Highway in Santa Monica when it slammed into the back of an 18-wheeler around noon. The Porsche is totaled. The passenger window was shattered and the bumper was fully off her car.

We're told she did not go to the hospital by ambulance, although we are told an ambulance was called to the scene.

No word on her condition, although we're told she seems okay. Doctors are performing tests.

2:35 PM PT -- Law enforcement sources tell us Lindsay and the truck driver were not under the influence of alcohol.



2:14 PM PT -- This is crazy ... sources tell TMZ, the Porsche wasn't even Lindsay's ... IT WAS A RENTAL!!!