Dallaglio is the first signing to be announced for BT Sport’s exclusive coverage of up to 69 live matches a season from the Aviva Premiership.
He will feature both as a co-commentator for some live matches and studio pundit for other games, when BT Sport launches in the summer.
“BT has huge ambitions for rugby union and I am delighted to be involved with that from the beginning,” said Dallaglio.
“I hope that we can make BT Sport the place to watch rugby and I know it has some great new ideas to really put the viewer at the heart of the action.”
Bruce Cuthbert, director commercial customers at BT Sport, said: “Rugby has great appeal, and for many pubs, clubs and bars it is even more popular than football.
"Providing both the best and most comprehensive coverage backed up with a great team of experts is critical, Lawrence is a giant of rugby union and we are thrilled that he has agreed to play such a pivotal role across our rugby coverage.”