UFC officials announced the 155-pound fight Friday during a Summer Kickoff news conference. The five-round fight will headline TUF 25 Finale inside T-Mobile Arena.
Gaethje (17-0) signed with the UFC earlier this month. The 28-year-old lightweight recorded five 155-pound title defenses during his time with WSOF and was widely viewed as one of the most talented fighters competing outside the UFC. He trains out of Denver.
Both Gaethje and the UFC were interested in booking his debut against the surging Edson Barboza (19-4), but the Brazilian lightweight is expected to undergo minor shoulder surgery next week and is unavailable to fight in July.
Johnson (17-11) has suffered losses in three of his last four fights, but is nevertheless considered one of the most dangerous lightweights in the UFC. He trains out of South Florida.