The 28-year-old said that he was looking forward to working under coach Michael Bradley at Connacht.
"I'm excited to be joining a Connacht side which is very competitive and gaining great impetus under the guidance of Michael Bradley and Eric Elwood," Campbell admitted.
"Obviously with Michael Bradley at the helm, I'm sure he'll have a significant influence on how I play, having been a former Ireland scrum half himself."
"Connacht is a squad with great potential and I'm looking forward to getting the opportunity to play again with former team-mates, having grown up with Adrian Flavin at London Irish and of course Johnny O'Connor and Gavin Duffy for Ireland."
Connacht CEO Gerry Kelly is delighted to sign such an experienced player and is impressed with Campbell's ambition.
"Kieran sees this as an opportunity to push himself back into the Irish set-up. While playing at Ulster he has been frustrated by the lack of playing opportunities due to the success of Isaac Boss and, with that in mind, he is a great asset for us to acquire for next season," said Kelly.