Gucci Mane's Prison Sentence Cut By 1 Year -- Huge Legal Win

Gucci Mane is a happier prisoner today ... after being told he'll be leaving the big house 12 months earlier than expected.

According to court docs ... after initially ruling against Gucci, an Atlanta judge has finally given the rapper the year's worth of credit for time served he's earned since his arrest for gun possession last September.

His 39-month sentence was originally scheduled to begin August 2014 when the judge made his ruling ... but now that sentence will begin at the time of his arrest.

The 34-year-old's been in and out of prison most of his adult life ... with prior stints for hitting a fan with a bottle and pushing a woman out of a moving car. He was even charged with murder in 2005 ... but prosecutors ultimately dropped the case.

He's scheduled to be released in December 2016.