EXCLUSIVE

Ex-Oakland Raiders star CB Sean Smith is slated to be released from jail next month -- after serving less than half of his year-long sentence in his street fight case ... TMZ Sports has learned.

As we previously reported ... Sean got 1 year in jail and 5 years probation after pleading guilty to felony assault in a July 2017 incident where he beat the hell out of his sister's boyfriend in Old Town Pasadena. The victim suffered several major injuries in the attack.

But now, officials have set his release date as Sept. 5 -- about half a year earlier than sentenced -- citing good behavior and overcrowding.

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Of course, Smith could screw it all up if he doesn't keep his nose clean -- but we're told if all goes smoothly, Smith will be a free man ... 1 day before the NFL season kicks off.

The timing is a huge deal for Sean -- who signed a $40 million contract with the Raiders in 2016 but was cut later after the attack -- because it may give him a chance to make a squad this year.

We've reached out to the NFL to find out if Smith will have to serve a suspension (he most likely will) ... or if he'll be eligible at all to play in the 2018 season.