UFC officials confirmed the bout to ESPN.com on Monday. The UFC 201 pay-per-view will take place July 30 at Philips Arena in Atlanta.
Namajunas (5-2) is coming off a unanimous decision win against Tecia Torres in April. Fighting out of Denver, the 23-year-old came up short against Carla Esparza in the UFC's first-ever 115-pound title fight in December 2014, but has won three in a row since.
ESPN.com ranks Namajunas and Kowalkiewicz the Nos. 3 and 7 strawweights in the world, respectively.