Unions plan to mobilise Wal-Mart workers around the world in an effort to secure more labour rights, amid threats of stoppages at some stores.
Wal-Mart says most employees don't want to join unions
Union Network International is pressing for more union representation at Wal-Mart stores and improved staff pay.
Unionised staff at German stores could stage walkouts as part of a global effort to put pressure on the retailer.
Wal-Mart does not recognise unions in the US but says that staff overseas are free to join a union if they choose.
Wal-Mart workers in Germany, Brazil and Argentina have joined unions, but labour organisations claim the retailer has resisted union demands elsewhere and that its general hostility towards unions has deterred union activity.
At its annual conference in Chicago, Union Network International said it would launch a concerted global campaign to increase worker representation at Wal-Mart stores.
Countries targeted by UNI - which represents 15 million workers in the services sectors - include South Korea, Mexico, Japan and the UK as well as Brazil, Argentina and Germany.
The action, which could include some stoppages at German stores, is designed to end what the union described as a "downward spiral" in the pay and working conditions of Wal-Mart staff.
UNI wants Wal-Mart and other global retailers to guarantee minimum labour rights at their various businesses and to give workers the right to unionise if they so wish.
Wal-Mart says unions are not necessary at its US stores
"These corporations often have bigger turnovers than substantial countries and they are too important to be allowed to operate without enforceable standards," said Philip Jennings, UNI's general secretary.
"They have got to stop cheating on their workers and guarantee labour rights and decent work, as determined by the International Labour Organisation."
Wal-Mart has always maintained that it has no official ban on unions.
However, the retailer discourages unionisation in the US, arguing that the "environment of open communications" in place at the company means there is no need for outside representation.
Wal-Mart stressed that it was seen as a model company in many of the countries in which it operated.
"Our policy is to operate within regulatory and customary business practices everywhere we operate," said spokeswoman Beth Keck.
"Our employees can choose whether they want to be union members or not and overwhelmingly they have not."
With 1.6 million staff, Wal-Mart is one of the largest private sector employers in the world.