Production on 22 new episodes begins next spring, and they will run in two batches. Last season's original episodes averaged 1.9 million adults 18-49.
The show, which launched in July 2004, will continue with Denis Leary, Peter Tolan and Jim Serpico as executive producers and show runners.
"The show has been a great source of pride to FX and remains one of the cornerstones of our respected brand of original series," FX Networks president John Landgraf said in a statement.
FX premiered a fifth season of plastic-surgery drama Nip/TuckTuesday at 10 p.m. to 4.3 million total viewers and 3.5 million adults 18-49, making it the most-viewed cable show of the night and No. 2 in all of TV in the 18-34 demo, second to Fox's House.