Welsh Rugby Union Group Chief Executive, Roger Lewis, said: "We are delighted to welcome Italy into this great competition.
"For the last two years it has been our ambition to deliver an enhanced tournament which will expand and continue to develop as a showcase for some of the very best of northern hemisphere rugby.
"It will prove a great incentive for all four Welsh Regions to test themselves against teams from a nation which is emerging as a real strength in the European game.
"Moreover, involvement in the Magners League will serve to strengthen Italy's position in the RBS 6 Nations tournament.
"The new, twelve team Magners League format across four nations will provide players, supporters and television viewers with an opportunity to savour some great fixtures.
"Our players already enjoy great rivalry with Italian opposition both internationally and through the existing cup competitions so the time is right to extend and develop that link through the Magners League.
"The inclusion of two Italian superteams will also add a new and important European dynamic to a competition which has become a powerbase of the professional game.
"I personally want to thank my colleagues from Celtic Rugby and all the other unions involved for their hard work and resolve to achieve this.
"I particularly want to thank Stuart Gallacher of Regional Rugby Wales who has worked tirelessly alongside me during the ongoing debate and discussions.
"Throughout the year long review and the more recent negotiations we have remained determined to ensure we achieved a great outcome for northern hemisphere rugby.
"This announcement will start an important new chapter for Welsh rugby and will will serve to encourage further support and widen the appeal for our national sport."