The duo announced dates for their UK tour last month, which included two shows at London's Brixton Academy (June 5 and 6) and a headline gig at Manchester's O2 Apollo (June 10).
They will now play a third show in Brixton on June 7.
Tenacious D is comprised of Jack Black and Kyle Gass, both on vocals and lead guitar.
Tickets go on sale tomorrow (April 20) here.
Tenacious D will release their third studio album Rise of the Fenix on May 15.
Listen to Tenacious D's 'Rize of the Fenix' below (Warning: Contains strong language):