For a struggling music industry, it provided other benefits as well, like boons for songwriters and publishers of featured music. In 2008, Sony/ATV, which publishes most of the Beatles’ catalog, negotiated a lucrative deal to have the show feature the group’s songs for two full weeks.

Image Jennifer Hudson in 2004. Credit Ray Mickshaw/Fox, via AP

And the annual concert series titled “American Idol Live,” which has been running since 2002 with contestants from the show, has grossed at least $217 million in ticket sales, according to the trade publication Pollstar.

The biggest-selling “American Idol” star is Carrie Underwood, the fourth-season winner, who has sold about 15 million albums, won seven Grammys and in December 2013 sang the role of Maria Von Trapp in NBC’s live broadcast of “The Sound of Music.” Kelly Clarkson, who won the first season, has sold nearly 14 million albums and remains a major attraction; “Piece by Piece,” her seventh studio album, opened at No. 1 on Billboard’s chart in March.

But in recent years the show has provided diminishing returns in record sales. Caleb Johnson, for example, who won the 13th season in 2014, opened at No. 24 with his album “Testify,” and in its second week the album fell to No. 131.

Image Kelly Clarkson on stage during the 2002 "American Idol" finale. Credit Fred Prouser/Reuters

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.