The 32 year-old left Ravenhill in the summer of 2012 and moved to London Irish where he has made 38 appearances, scoring 212 points.
Humphreys played for Ulster at Under 21 level as well as representing Ireland at Under 19 and Under 21 level before moving the Premiership to join Leicester Tigers. Following three seasons at Welford Road he returned to Ravenhill in 2008 and made 110 appearances for Ulster scoring 617 points.
Speaking about his decision to return to Ravenhill, Humphreys said:
"I have loved my time at London Irish but when the unexpected opportunity to come home arose, it was a very easy decision to make. My wife and I are very much family people a nd we enjoy being in Northern Ireland.
"It is exciting to be coming back to Ulster, to be part of a squad that is challenging in Europe and that is at the top of the RaboDirect PRO12 every year. As well as a strong team, Ravenhill has been transformed since I left two years ago. Everyone in the Premiership has heard about it and seen pictures of the new facilities. It is just phenomenal how good the new stadium is.
"I am realistic. I know that Paddy Jackson is one of the best young players in Ireland, but I am competitive. It is up to me to push him and hopefully that will bring out the best in both of us."
David Humphreys, Director of Rugby at Ulster, added:
"With more than 60 appearances in the Premiership and over 100 caps for Ulster, Ian brings significant experience to our squad in the highly important position of outhalf.
"He has learned a great deal in the past two seasons working with coaches like Brian Smith and S haun Edwards and that knowledge can only benefit the young players we have coming through.
"With Paddy Jackson playing an increasingly important role with Ireland, Ian gives us another option at outhalf, something which is highly important if the team is going to be successful in the future.
"I am delighted that he has rejoined Ulster."
Guinness PRO12
Suite 208, Alexandra House,
The Sweepstakes
Ballsbridge, Dublin 4, Ireland