Lansdowne were crowned Division 1A champions for the first time on Easter Saturday, following their 32-25 bonus point win over second-placed Clontarf.
Earlier this year, out-half Ronaldson brought his excellent Ulster Bank League form to Club International level when he was named, for the second year running, in the Ireland Club team.
The former Kilkenny College captain, now aged 23, landed six of his seven shots at goal for a 15-point haul in Ireland's richly deserved 30-20 Club International victory over England Counties.
Ronaldson will join up with Connacht for pre-season training in June. He said: "I'm delighted to have been given this opportunity with Connacht Rugby.
"I've really enjoyed my rugby with Lansdowne this year and I'm looking forward to the chance to develop further as a player with a province that is really going places.
"There are a lot of changes at Connacht with a new head coach in Pat Lam coming in and I'm excited about the move to Galway and linking up with everyone there."
Commenting on the westerners' latest signing, Connacht CEO Tom Sears said: "Craig is a talented young player who has earned his opportunity in the professional game.
"He has consistently demonstrated in the Ulster Bank League that he has all the ability to make the step up and we have no doubt he will thrive in a professional environment.
"He is a player we have watched closely for some time and he now has an opportunity to develop his undoubted talent and fulfil his potential."