The WalMart Supercenter signage is seen in Springfield, Ill., Monday, May 16, 2011.Seth Perlman/The Associated Press
A spokesman for the South Africa Commercial, Catering and Allied Workers Union (SACCAWU), said it had filed a notice of appeal with the country's Competition Appeal Court.
Such a filing is an initial step in the appeal process, though both retailers said the move would not halt the deal and one investor was skeptical about the union's chance of success.
The world's largest retailer finalized on June 20 its purchase of 51 per cent of discount retailer Massmart, after winning approval with minimal conditions from South Africa's Competition Tribunal.
"We are unhappy with the decision of the tribunal," Lucas Ramatlhodi, a co-ordinator and secretary for SACCAWU, told Reuters. "The conditions it put on the merger are not binding and they are weak and too ambiguous."
Under the conditions, Massmart must not cut jobs for two years, honour existing labour agreements and work to develop local suppliers. Unions had been pushing for strict targets on using local manufacturers.
It was not immediately clear whether the union would be able to force a delay or additional conditions on the deal.
"This is not an unexpected development and the companies remain firm in their belief that the transaction rests on solid legal ground," Wal-Mart and Massmart said in a joint statement.
"SACCAWU's notice of appeal has no impact on the implementation of the transaction."
Shares in Massmart remained buoyant, and were up nearly 1 percent at 1217 GMT, roughly in line with Johannesburg's Top-40 index of blue-chips.
"It can create uncertainty in future if there's a reason to believe that the deal can be derailed. I believe it's not possible," said Abri du Plessis, Chief Investment Officer at Gryphon Asset Management
Massmart said on Sunday it aims to create 15,000 jobs in South Africa and increase its procurement of food and consumer goods by an additional 60 billion rand ($8.8 billion) over the next five years.
Wal-Mart's bid was seen as a test for foreign investment in South Africa, which is home to both the continent's deepest capital markets and powerful unions.