Born in Edinburgh and educated at Cheltenham College and Oxford University where he studied Philosophy and Theology and won two rugby union Blues, Danielli represented England at Under-21 and Students level before opting to play for Scotland.
He made his full international debut for the country of his birth against Italy in 2003, ahead of the start of the World Cup and won his first start for Scotland during the World Cup tournament against the USA, scoring two tries.
He made his Six Nations debut at Murrayfield against England in 2003 and since then, he has been a regular member of the Scotland squad.
Ulster coach Mark McCall said: "We are very pleased to be able to sign a player of Simon's quality and proven pedigree. Although he is a very experienced player, having represented Scotland, Bath and the Borders, at only 27 years of age we feel he still has his best years ahead of him and we are confident we will see them at Ravenhill."
Danielli added: "I'm really excited and honoured to be joining Ulster, the fans are terrific and the atmosphere at Ravenhill is one of the best I've ever experienced. I'm looking forward to playing in a team with so many world-class players and such strong ambitions."