An important retail industry metric, the same-store number measures the change in sales at stores that have been open at least a year.
The world's largest retailer had predicted a sales gain of 1% to 3% for its August period, which extended from July 29 through Aug. 25.
The August gain exceeded a 2.4% increase in July.
Wal-Mart will report full August results on Thursday, when other major retailers also plan to release monthly sales.
Shares ended Friday up 18 cents at $43.88.