The company behind Costco's Kirkland Signature hard seltzer seems to be beverage developer Patco. As of 2020, the address on the seltzer’s packaging matches that of Patco Brands.

On its website, Patco is described as "a leader in private label beverage development and production." It's also linked with brands like Target, Walmart, Whole Foods, and Kroger.

Patco also has relationships with brands like Rancho La Gloria (which has a line of hard seltzers) and Miracle Seltzer (which has made hard seltzers in partnership with 7-Eleven).

Patco is certainly familiar with the hard seltzer game. Many Redditors have likened the Costco product to White Claw, which is currently one of the best-selling hard seltzers.