His departure follows the retirement last month of Chief Justice Randall Shepard.
Sullivan was appointed to the five-member court in 1993 by Democratic Governor Evan Bayh.
Current Governor Mitch Daniels, a Republican, will select Sullivan’s replacement from candidates selected by the Indiana Judicial Nominating Commission.
Sullivan says he’ll remain on the high court until the start of the law school’s fall semester.
(Information for this story came from the Associated Press)