Sheriff's Office spokeswoman Tracy Flanagan says Willie Tiller, a former cellmate of Nichols, suffered non-life threatening injuries.
Tiller testified for the prosecution in the Nichols trial Tuesday. He told a jury Nichols was angry at the judge in his 2005 rape case. Tiller is serving a 10-year sentence for armed robbery.
Flanagan would not comment further, citing a gag order.