Former NFL safety Sergio Brown is fighting back in his murder case this week ... pleading not guilty to charges that he killed his mom and concealed her body afterward.

According to a spokesperson for the Cook County State's Attorney's Office, Brown entered the pleas before an Illinois judge on Wednesday.

He's now due back for another hearing in the case late next month.

As we previously reported, Brown was first hit with one charge of first-degree murder and one count of concealment of a dead body back in October ... after officials claimed he killed his 73-year-old mom in Maywood in September and dumped her body near a creek behind her home.

Authorities alleged that following the killing, Brown hightailed it to Mexico -- where he was seen partying and dancing seemingly without a care in the world at a Tulum club.

Brown was eventually extradited and arrested ... and later transported to Illinois to face the charges.

He's currently being held behind bars without bail.