The 27-year-old former Bath and Bristol back joins Ulster from Borders, whom he joined in September 2004.
Danielli was in the provisional Scotland squad for this year's RBS 6 Nations but has not featured in either of their opening
He said: "I am 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."
Ulster Coach Mark McCall added: "We are very pleased to be able to sign a player of Simon's quality and proven pedigree."