The Moonlighting actor will play cunning attorney Peter Goldman in the TNT drama's final run, according to TV Guide.
When her gangster son is killed, a distraught mother will hire Goldman to sue Brenda Leight Johnson (Kyra Segdwick) and her colleagues at the LAPD.
"He's going after her, the city and the department," explained series creator James Duff. "He is a wily attorney who's made a profession of suing the police.
"He would say he's a necessary part of the justice system. He's making the police justify their tactics and holding them to the standards our constitution demands."
Lost star Mark Pellegrino will also appear in the new season of The Closer as a defence attorney hired to combat Goldman in court.
Pellegrino recently played the role of Bishop in the first season of Syfy's Being Human and has played recurring roles on Supernatural and Dexter.
The Closer returns to TNT on Monday, July 11 at 9/8c. Armstrong will make his first appearance in an episode expected to air on August 8.
> Kyra Sedgwick explains The Closer exit