Austin, Texas-based GSD&M has worked with the retail giant for almost 20 years and Kansas City, Missouri, agency Bernstein-Rein, which also handles media buying, for more than 30 years.
The review, which has been confirmed by Wal-Mart, could see the retail giant centralise its account or leave it split between two shops.
A spokeswoman for Wal-Mart said: "We could choose one. We could choose many. It's too early to know. It might include one incumbent or both.
She stressed the importance of working with agency partners that were able to deliver fully integrated communications across all of the media that the retail behemoth uses.
She added that a search consultancy had not yet been appointed and there was no time scale available on the pitch process. "All we've done so far is just notify the agencies."
There has been much speculation about a possible US review since the arrival of chief marketing officer, John Fleming, who joined from rival retailer Target.
Fleming's arrival has seen Wal-Mart try to throw off its low-price message and move upmarket.
Earlier this year, it launched a new tagline to underscore this approach with "Look beyond the basics".
The ads feature testimonials from shoppers who while shopping for basic items are surprised to find much more, such its line of clothing Metro 7, which targets hip, urban, fashionista women with a cheap but chic line.
For the launch of Metro 7, Wal-Mart has run ads in Vogue, fronted by former Miss Universe Dayanara Torres, and ex-wife of pop star Marc Anthony, to shout about the line and to change perceptions.
Underlining its ambitions in fashion further was a reported bid last year for fashion brand Tommy Hilfiger.
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the .