The network announced Harmon's one-year contract extension on Friday along with the renewal of a show that just keeps getting bigger.
Now in its 10th season, "NCIS" for the first time ranks as TV's most-watched program, CBS said, outpacing even NBC's "Sunday Night Football." "NCIS" logs an average 21.5 million viewers each week, up 7 percent over last year.
Harmon stars as Special Agent Jethro Gibbs, who heads up a team that investigates crimes involving U.S. Navy and Marine Corps personnel. The cast also includes Michael Weatherly, Pauley Perrette, Rocky Carroll and David McCallum.
"NCIS" premiered in Fall 2003.