The Dolphin Tale star will take on the role of Executive Assistant District Attorney David Hadena, whose work relationship with Olivia Benson (Mariska Hargitay) leads the pair into a romantic relationship.
NBC Entertainment executive Bob Greenblatt announced the casting during the Television Critics Association Press Tour on Friday, offering reporters a sneak peek at the duo on the Law & Order: SVU set.
"This is a home run on so many levels," said Hargitay. "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."
Hargitay also confirmed her intention to continue with the series into next season, saying that she is having "too much fun" to consider leaving. She recently took on a reduced role in the drama in order to spend more time with her family but will still appear in every episode of the 13th season.
In December, filming on Law & Order: SVU was halted by the Occupy protests when demonstrators prevented access to key shooting locations.
The first instalment of Connick Jr's four-episode arc will air on January 18.
Watch Harry Connick Jr and Mariska Hargitay on the Law & Order: SVU set below: