Sprint Corp. has moved its $700 million media agency business from Publicis Groupe’s PUBGY -4.58% Mediavest Spark to independent media agency Horizon Media in one of the largest account changes of the year.

The win gives Horizon Chief Executive Bill Koenigsberg another feather in his cap as he expands his independent media operation. Last year, he joined forces with Hyundai-backed agency Innocean to launch another media agency called Canvas Worldwide. As part of the deal, Canvas also won the media account for Hyundai Motor Group’s U.S. brands, including Kia Motors America and Hyundai Motor America.

“Horizon Media’s innovative ideas and dynamic approach using traditional and emerging channels is a perfect fit for Sprint as we continue on our transformation,” said Sprint’s marketing chief, Roger Solé, in a statement. “The agency’s unique and fresh perspective will support Sprint in an extremely competitive and continually evolving industry.”

Wireless companies have been locked in a fierce price war that only threatens to get more competitive as cable companies like Comcast and Charter Communications prepare to enter the cutthroat business.

Sprint spent just under $700 million on U.S. media in 2016, compared with spending by AT&T Inc. of $1.62 billion and Verizon at $1.3 billion, according to Kantar Media. The spending figure doesn’t include some forms of digital advertising. All three spent less on U.S. media in 2016 than in 2015.

Sprint, the No. 4 U.S. carrier by subscribers, has aggressively gone after its larger telecom rivals in recent years with offerings like unlimited data. But AT&T and Verizon recently launched unlimited data plans, forcing Sprint to end its longtime offer to charge customers half the price of other rival plans.

The intense competition has been clear even in the companies’ creative approach to their advertising. Last year Sprint tapped Paul Marcarelli, Verizon’s longtime “Can you hear me now?” pitchman, to star in ads in which he boasts about switching to Sprint.

Sprint’s move to independent media shop Horizon is also in line with the phone company’s recent creative agency shift from Interpublic Group of Cos .-owned Deutsch to Droga5, also an independent agency.

For Publicis Groupe, the departure of the Sprint account is another blow after a string of large account losses in recent years, including Wal-Mart, Procter & Gamble, Coca-Cola and Honda. Publicis also recently won some business, including H&R Block, KFC and P&G in the U.K.

“We are proud of the work we’ve delivered for Sprint during our partnership with them and our team’s dedication to the client,” according to a statement from Mediavest Spark. “We wish Sprint all the best in the future and will approach this transition with the utmost commitment and collaboration.”

Write to Alexandra Bruell at alexandra.bruell@wsj.com