The company behind these products is Olde Thompson, which is based in California (it manufactures some of the other spices for the chain, too). Olde Thompson has been in business since 1944 and it manufactures not only house-brand spices for retail partners, but a variety of spices and seasonings under its own brand as well. Because Olde Thompson processes and packages the black pepper for Costco, the grocery store chain can sell its black pepper products for a terrific price; $8.73 per pound for its whole black peppercorns.
The Company Behind Costco's Kirkland Brand Black Pepper

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Olde Thompson has been Costco's go-to partner for whole black peppercorns (which are just one of the many different types of peppercorns) since at least 2018. In fact, you might remember that, around this time, the label on Costco's pepper products switched from proclaiming that its pepper was Tellicherry black pepper sourced from India to black pepper sourced from Vietnam. While this switch in sourcing might have initially come as a shock to customers, sourcing peppercorns from Vietnam makes sense; the country actually produces more pepper than any other country in the world thanks in part to its ideal climate.
Some have speculated that the switch was a cost-saving measure made by Costco. What's more customers have complained about the product on the company's website saying, "the 'Tellicherry' peppercorns they used to sell had a better aroma, and flavor." Other customers have shared similar sentiments, noting that the Tellicherry peppercorns were also larger.
Contrary to popular thought, Tellicherry peppercorns do not have to come from Tellicherry, India. In fact, Tellicherry peppercorns are regular black peppercorns that can be grown anywhere. The only stipulation is that they have to be at least 4.25 millimeters in diameter. Interestingly, one enterprising Redditor discovered that, in their container of Kirkland Signature Whole Black Peppercorns (produced by Olde Thompson and sourced from Vietnam), over 75% of the peppercorns met this size grade and thus could be considered Tellicherry peppercorns. So, it turns out this Vietnamese black pepper isn't all that different to the Indian product Costco customers were so enamored with.