Disney Star Billy Unger -- Cops Plea in DUI Case ... I Was Wet Reckless

Billy Unger from the Disney show "Lab Rats" ... plead no contest to a lesser charge in his DUI case -- keeping him out of jail as long as he toes the line.

As TMZ first reported -- Unger was nailed for speeding in Malibu back in June, but the then 18-yr-old also blew a .08 ... which is the legal limit if you're 21. Under Cali law minors behind the wheel can't have a drop of alcohol in their system.

Prosecutors offered Unger a plea deal -- dropping DUI and speeding charges ... and he plead no contest to wet reckless. It's a lesser charge usually offered to first time DUI offenders like Billy who were at the legal limit.

Unger got a fine, 2 years probation, and has to complete alcohol education programs.