Walmart is ending its nearly decade-long relationship with Publicis Groupe's Mediavest, the company confirmed.

"We will continue to work with Mediavest during the transition," said a Walmart spokeswoman, who declined to say what that transition will be for its North American media business.

Walmart is ranked No. 13 on Ad Age's list of the top media spenders. The retail giant, including its Sam's Club brand, spent over $900 million on measured media in the U.S. in 2014, according to the Ad Age Datacenter.

The move followed the departure of Walmart U.S. Chief Marketing Officer Stephen Quinn on January 31. Tony Rogers was named his successor as CMO in December and took the helm earlier this month.

"We're proud of the talent dedicated to this business and the award-winning, results-driven innovation we delivered for Walmart over the years," said a Mediavest spokeswoman. "We continue to partner with Walmart as they transition to their new model and wish them the best in their future direction."

For Publicis Groupe's Mediavest, the loss couldn't come at a worse time. The shop in 2015 lost Procter & Gamble's U.S. buying, as well as its massive Coca-Cola account.

Brian Terkelsen continues to run Mediavest in the U.S., but the shop lacks a dedicated global CEO following a massive holding company reorganization that moved Starcom Mediavest Group Global CEO Laura Desmond into a holding company role overseeing chief client officers. Pubilcis' media networks, including Starcom Mediavest Group and ZenithOptimedia, have been rolled up into one Publicis Media hub overseen by ZenithOptimedia CEO Steve King.

Walmart in January 2007 awarded the media account to MediaVest and the creative account to Interpublic Group's Martin Agency. Those moves followed a series of events after Walmart in May 2006 put its accounts in review. In November 2006, it selected Interpublic's DraftFCB for creative and Aegis' Carat for media. But just a month later, it terminated Julie Roehm, senior VP-marketing communications and point person on the review, and put the accounts back up for grabs.

Walmart's ad spending rate has grown considerably over time, from just 0.30% of net sales spent on advertising in the year ended January 2001, to around 0.50% since fiscal 2012 ending in January.

In more recent years, Walmart has brought more of its digital buying in-house as it's invested in its own digital targeting, buying and optimization platform, called Walmart Exchange. It's unclear what this latest move means for its in-house buying operation.

The move follows an unprecedented number of reviews and agency changes by big marketers in 2015, including Coca-Cola, L'Oreal, GSK, Johnson & Johnson, Procter & Gamble, Unilever, Volkswagen and Sony, among others.

Contributing: Jack Neff