Clinton Portis could be headed to prison for a LONG time ... 'cause he's now facing up to 10 years behind bars after he agreed to plead guilty to a fraud charge last week.
According to court docs, obtained by TMZ Sports, the former NFL star cut a deal with prosecutors on Friday ... and pled guilty to one count of conspiracy to commit health care fraud.
As a result, per the docs, 40-year-old Portis now faces a maximum sentence of 10 years in prison ... as well as HEAVY fines.
Portis had been hit with multiple charges by federal prosecutors in 2019 ... after prosecutors say he -- and several other ex-NFL players -- developed a scheme to get cash from the Gene Upshaw NFL Player Plan.
Prosecutors say Portis and the other former players made up fake claims and submitted them to the plan, which was initially designed to help retired players pay for medical bills.
Prosecutors alleged Portis and the players made huge coin after the plan reimbursed them.
Investigators say some of the fraudulent claims the players were reimbursed for included pricey hyperbaric oxygen chambers and ultrasound machines.
Portis went to trial over the allegations last week, but jurors could not reach a verdict. The case had been set for a re-trial ... before Portis ultimately reached his deal with prosecutors.