"As many suspected, I can finally confirm: I have been invited by Queen to sing one very special concert! Sonisphere at Knebworth July 7th!" he wrote.
Earlier this month, Lambert tweeted that his remarks in the UK's Daily Star on paying "tribute to Freddie [Mercury]" at the event had been taken out of context.
"Sorry to be coy about the details the past few weeks, but I made a promise to keep it under wraps as best I could," he added on Monday. "Gonna be a surreal night!"
The "American Idol" alum has lent his vocals twice to Queen's remaining lineup, performing with the band at the 2009 "Idol" finals as well as a November show at the MTV EMAs last year.
Sonisphere is the site of the band's final 1986 performance with the late Mercury. The rocker died in 1991.
The summer date will arrive after the launch of Lambert's next solo album, "Trespassing," in March.