The heavy metal titans will appear at the festival in Knebworth, Southern England, between July 4 and 6 next year.
The performances will be the only shows by either band in the UK in 2014, and will mark 40 years of rock concerts held at the venue.
Iron Maiden vocalist Bruce Dickinson said: ''We all grew up in England well aware of the importance of the legendary concerts at Knebworth. Without being cliched, it really is an honour and privilege to return to play there.''
Bruce also said the date - the last in the band's Maiden England World Tour - will see them play some songs for a final time.
He added: ''If you are there on the day it is highly likely that you will hear some of these songs played live for the very last time ever.
''So on an occasion like this, a celebration and the end of a chapter, you can be sure we will do our utmost to give our fans one hell of a show and go out with all guns blazing.''
Meanwhile Metallica will let fans pick their set list by voting online ahead of the concert.
Drummer Lars Ulrich said: ''What could be more fitting on our third Knebworth go-around than to play an all-request set, chosen entirely by the fans? We are so excited about the limitless possibilities of our friends picking what songs Metallica get to perform on this forthcoming evening from the nearly 140 songs we've recorded.
''Add the cherry on top...co-headlining with Iron Maiden, who are a big reason that Metallica exists in the first place. Although we haven't played together on the same day since 1988 ... this will hopefully amount to what many rock fans would consider a dream bill.''
Further bands will be added to the line up closer to the date. For more information visit Sonisphere.co.uk
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