Analysts polled by Thomson Reuters expected a 1.8 percent increase.
This figure is a key indicator of a retailer's health because it excludes results from stores recently opened or closed.
The metric climbed 5 percent in the U.S. and edged up 1 percent overseas for the five weeks ended Jan. 5.
Stripping out lower gas prices and foreign currency fluctuations, sales at stores open at least a year increased 5 percent. The figure also increased 5 percent in the U.S. and rose 7 percent abroad.
Total monthly revenue for the Issaquah, Wash., company climbed 6 percent to $11.53 billion.
For the 18-week period, sales at stores open at least a year increased 3 percent. In the U.S. the metric rose 4 percent. It climbed 1 percent internationally.
Removing lower gas prices and foreign currency fluctuations, Costco Wholesale Corp. said that sales at stores open at least a year gained 5 percent. The figure increased 4 percent in the U.S. and climbed 7 percent overseas.
Total revenue rose 6 percent to $36.26 billion.
Last month Costco reported that its fiscal first-quarter net income rose 2 percent as sales improved and the wholesale club operator pulled in more money from membership fees.
Costco currently runs 648 warehouses, including 461 in the U.S. and Puerto Rico, 87 in Canada, 33 in Mexico, 25 in the U.K., 18 in Japan, 10 in Taiwan, nine in Korea and five in Australia.
Costco shares rose $1.51, or 1.3 percent, to $115.56 in premarket trading about 3 ½ hours ahead of the market open.