Its waning influence in the music world reflected its decline on TV. As its format grew old and it faced increasing competition from other talent shows, “American Idol” suffered a steady drop in viewers, especially in the 18- to 49-year-old demographic that is most attractive to advertisers. The current season has drawn just a 2.1 Nielsen rating in that age bracket, down from the 12.6 rating it recorded in season 5.

The last season of “American Idol” will begin in January, Fox said Monday. The longtime host, Ryan Seacrest, will be joined by the judges Jennifer Lopez, Keith Urban and Harry Connick Jr.

Mr. Newman said the final season would be a “true season-long celebration.”

“We really wanted to do it in a way that felt special and celebratory and treat the show the way it deserved to be treated,” he said.

Image Adam Lambert performing on "American Idol" in 2009. Credit... Frank Micelotta/Fox

Fox made the announcement about “Idol” as it unveiled its fall lineup in a format known as the upfronts. The network said that another old Fox hit, “The X-Files,” would return as a limited series in January, and its first episode will be shown after Fox’s broadcast of the always highly rated pro football N.F.C. championship game. The stars of the original show, David Duchovny and Gillian Anderson, will both return.

The biggest new hit on TV this season, “Empire,” will return on Wednesdays at 9 p.m., and begin again this September, Fox said. The network said that it would show 18 episodes next season, up from the 12 this year, and that it would split the season in two.

Fox, which is the fourth-rated broadcast network, will introduce a series of new shows after several flops last season. The new lineup includes “Lucifer,” which is based on a DC Comics series, and “Minority Report,” based on the futuristic 2002 movie starring Tom Cruise.