Connick will play Executive Assistant District Attorney David Haden, a "dedicated, straight-shooting prosecutor" assigned a case with Detective Olivia Benson, played by Mariska Hargitay, the network said.
"Although Benson is at first wary of Haden, they are both surprised to find they work well together. As the case develops, so does their relationship," a synopsis said.
Connick's first episode is to air Jan. 18.
"This is a home-run on so many levels," Hargitay said in a statement. "The show is very fortunate to have Harry's extraordinary talent, and I'm lucky because I get to work with my friend. I think Olivia couldn't have asked for a better companion to take her through a new stage in her life and career."
Connick Jr. currently is starring on Broadway in the musical "On a Clear Day You Can See Forever." His film credits include "Memphis Belle," "Copycat," "Hope Floats" and "Dolphin Tale."