We broke the story ... 20-year-old Musso was busted back in October after an early morning traffic stop in Burbank, CA.

Now, TMZ has learned Musso has pled no contest to one count of having a blood alcohol level over .08 ... and in exchange, will be sentenced to 36 months informal probation and ordered to pay a fine and complete an alcohol education class.

Musso released a statement ... saying, "I appreciate the court allowing me to resolve my case in the manner it did today. In becoming an adult, I have learned first hand that stepping up and taking responsibility is the best way to move forward."