The American funk-rock band sold more than 200,000 in four hours for their original cast of four open-air shows in Manchester, Edinburgh and two at London's Hyde Park.
Now they have decided to add the Welsh capital to their itinerary and will be playing the Millennium Stadium on Wednesday, 23 June.
Tickets for the two extra dates - they will also play a third night at Hyde Park on 25 June - will go on sale at 9.00am on Saturday, 21 February. The capacity is likely to be 64,000
"This is a fantastic coup for the Millennium Stadium and great news for Cardiff and Wales. The concert will come three days before we celebrate the fifth birthday of the first event at the stadium," said the Millennium Stadium general manager Paul Sergeant.
"The Millennium Stadium date will be one of the biggest on the band's tour schedule with a capacity of more than 60,000 and will once again demonstrate the stadium's ability to attract the top events to Wales.
"It will also be the first concert attracted to the stadium since we took the decision to bring the selling of major events at the stadium in-house and there are very encouraging signs for the future with a number of major promoters interested in bringing more acts to Wales."
The Red Hot Chili Peppers will follow in the footsteps of the Manic Street Peachers, Tina Turner, Bon Jovi, Robbie Williams and the Stereophonics, all of who have played at the Millennium Stadium.
Michael Jackson, the Rolling Stones, U2 and Dire Straits all played at the old Cardiff Arms Park before the Millennium Stadium was built.
The Millennium Stadium is also bidding to host the proposed one-off rugby fixture between the British and Irish Lions and France in the build-up to next year's tour of New Zealand.
And Paul Sergeant and his sales team are also in negotiation to stage a unique charity benefit match at the Millennium Stadium in June this year for two of rugby union's greatest names, Neil Jenkins and Jonah Lomu.
"The Lions have played in Cardiff before and we will be fighting hard to put together a winning bid to bring their special game with the French to Cardiff," said Sergeant.
"The Millennium Stadium can offer the perfect setting for such a prestigious fixture and it would be another major coup for Wales if we could win the right to host the game.
"Negotiations with representatives of both Neil Jenkins and Jonah Lomu are at an advanced stage and we are currently examining what players would be available to take part in a benefit match that would have a strong charity theme.
"We already have 23 major dates confirmed for the Millennium Stadium this year and we are working hard to increase both the volume and diversity of events we host."
Red Hot Chili Peppers Tour Dates
" Murrayfield Stadium, Edinburgh (June 13)
" City of Manchester Stadium, Manchester (June 18)
" Hyde Park, London (June 19-20, 25)
" Millennium Stadium, Cardiff (June 23)
23 June, 2004: CARDIFF, Millennium Stadium
Ticket price - Â£45, Â£35
Tickets available through: 0870 400 0688 / 0870 066 7799 / 02920 224488 Online at www.cclive.co.uk / www.ticketmaster.co.uk