A man accused of raping his teenage relative secured a sweetheart plea deal from Manhattan District Attorney Alvin Bragg — and then went on to allegedly sexually terrorize five people in the Bronx just a month later, The Post has learned.

Justin Washington, 25, was promised a plum 30-day jail sentence, along with five years of probation, after he agreed in August to plead down to a charge of coercion in his Manhattan rape case.

But just a week before he was set to be sentenced Wednesday, Washington allegedly went on a sex-crime spree, attacking four women and a man near the homeless shelter where he was living in the Bronx.

After finding out about the Sept. 15 arrest, prosecutors with the Manhattan District Attorney’s Office said they were reconsidering the sentence recommended with the plea deal — which some cops said should never have been offered in the first place.