Sales in stores open at least a year is a key metric of a retailer's health.
The San Francisco-based retailer said Thursday that the entire month was challenging, but its performance improved toward the Memorial Day weekend.
For the four weeks ended May 30, total sales fell more than 5 percent to $1.18 billion from $1.25 billion.
Gap has long been struggling, unable to get shoppers to buy its clothes without offering big discounts. chief executive officer Art Peck, who came to the helm in February 2015, has been trying to overhaul the business.
In after-hours trading, Gap Inc. shares rose nearly 5 percent to $19.18. For the year to date, shares are down about 26 percent.