NCIS” has turned into USA Network’s own Rose Bowl.

The cable net’s “NCIS” marathon on New Year’s Day helped give USA its second-highest rated day ever. From 6 a.m. to 6 a.m. the following day, USA averaged 3.1 million viewers.

USA’s highest-rated day ever? That came on Jan. 2, 2010 — when another “NCIS” marathon gave USA a full-day average of 3.15 million viewers.

Clearly the investment in “NCIS” has paid off bigtime for USA, which paid CBS TV Distribution a bargain $1 million for the show. Those stellar “NCIS” numbers are one reason that USA stepped up just weeks after the fall launch of spinoff “NCIS: Los Angeles” in 2009, preemptively paying CBS $2.5 million a seg to lock that show up too.

“There’s a tone to it that fits our brand,” said USA program research veepee Ted Linhart. “It’s not too heavy but heavy when it needs to be and has a good sense of humor, which is something I think a lot of people don’t realize. It’s easy to watch and uplifting.”

Linhart said USA has carved out a niche on New Year’s Day (and the day after) as a programming alternative to the college bowl games. In years past, the cabler has run marathons of shows such as “Monk.” But “NCIS” is a whole different beast.

CBS TV Distribution topper John Nogawski couldn’t help but gloat about the show’s boffo perf.

“‘NCIS’s” performance just goes to show that one of the best investments a cable network can make is acquiring a proven hit series,” he said.