A spokesperson with the Nassau County D.A.'s Office tells TMZ ... Lindsay Lohan's mom pled guilty to operating a motor vehicle while intoxicated and leaving the scene of an incident without reporting it.

We're told Dina will have to spend 18 days in jail and will then be on 5 years probation. She also must participate in a victim impact panel and take part in an alcohol program.

As we reported ... Dina was busted back in January after cops say an outing at Outback Steakhouse ended with her t-boning another car outside the restaurant and leaving the scene.

Cops ultimately tracked her down at her Long Island home ... where cops say she fell on her face and accused the responding officer of pushing her. Cops said they detected a strong odor of booze on her breath and noticed Dina had glassy, bloodshot eyes too.

BTW ... Dina had a prior DWI in 2013 also in Long Island.