Same-store sales at shops open at least a year -- a measure that strips out the volatility of openings and closings -- rose 4.1 percent last month from the same period a year earlier, according to Retail Metrics Inc.
The surge is the largest since sales swelled 5.1 percent in January. Retailers had struggled through the spring as they dealt with storms and chilly temperatures and "an economic soft patch," said the group's president, Ken Perkins.
June results beat forecasts of a 3.7 percent boost and also outperformed the 3.8 percent increase recorded in May and the 0.3 percent upswing in June 2012, he said.
The cause, according to Perkins: "An improving labor market coupled with falling gas prices during the month, seasonably warmer temperatures that drove summer clearance, rising home prices and generally better macroeconomic conditions."
Discounters did especially well. Fred's saw a 4.5 percent same-store sales increase last month.
Stein Mart Inc. said sales were up 6.5 percent due to strength in linens, ladies' casual sportswear and women's boutique goods. Menswear and female special sizes were less impressive.
The retailer, which has 262 stores, said Florida and other gulf states drew high revenue. California suffered, however.
But even the "extreme heat" on the West Coast toward the end of the month couldn't keep club chain Costco from reporting a 6 percent comparable sales boost. Nor could cannibalization from new stores, according to a report from Sterne Agee analyst Chuck Grom.
Traffic at the company was up 5 percent last month, which propelled strong sales in frozen foods and candy.
A separate account of retail sales from Thomson Reuters also showed companies beating Wall Street forecasts. Predictions of a 3.8 percent sales increase when including drug stores and a 4.8 percent surge without them proved inaccurate.
Thomson Reuters calculated that the 11 retailers in its roundup enjoyed a 4.2 percent uptick with drugstores and a 5 percent rise without.
American Apparel said its same-store sales swelled 7 percent in June. But most other clothing retailers were weak, with the category up 1 percent despite expectations for a 2.3 percent upswing.
Sales at Limited Brands were flat compared with 7 percent growth last June. After lower clearance sales, Victoria's Secret slipped 1 percent after an 11 percent rise last year, according to RBC Capital Markets analyst Howard Tubin.
Bebe Stores Inc., which does not disclose monthly sales data, said Thursday that its same-store sales for the fourth fiscal quarter tumbled 7.1 percent. The Brisbane, Calif., women's wear company said traffic declined at its 240 stores.
The brand also offered more promotions throughout the period as it tried to sell off excess inventory.
Retail sales reports were to be finalized after Gap Inc.'s data release, after the market closes.