Sebastian Telfair is now behind bars … TMZ Sports has learned the former NBA star has officially begun his prison sentence.

Records show the 40-year-old reported to Fort Dix FCI in New Jersey, where he’s expected to remain for the next six months.

A judge, as you know, ordered Telfair earlier this summer to the time in a cell … after he violated the conditions of his supervised release in his healthcare fraud case.

On Monday, he told us out in New York City he thought the whole situation was “super unfair,” and he actually pleaded with Donald Trump to pardon him.

“I did too good to be sitting in anybody’s jail,” he said.

The prison Telfair’s locked up at touts itself as “a low security federal correctional institution.” Its website says it has a current population of over 3,000 inmates.

It’s unclear if POTUS is currently mulling over a Telfair pardon … but the ex-basketball player told us, regardless, he’s going to use his time in prison to better himself.