He has spent the last two years playing for Ulster and joins the Connacht squad as new coach Eric Elwood puts the finishing touches to his playing panel for the upcoming Magners League.
The 25-year-old former Blackrock College student played for UCD before making his Leinster debut in 2006. He went on to earn 12 caps before moving to Ulster in 2007, with whom he played for 15 times.
Willis arrived in Galway in time to join the squad and fellow scrum halves Frank Murphy and Conor O'Loughlin on their trip to the Aran Islands where they are undergoing pre-season training.
Their pre-season stint is being supervised by Elwwod, assistant coach Dan McFarland and newly-appointed coaches Brian Melrose (backs) and Mike Forshaw (defence).