The 25-year-old follows former Dingwall team-mate Paul Lawson to Fir Park after penning a two-year deal.
Vigurs told Motherwell's official website: "I am absolutely delighted to be joining - Motherwell are a massive club who did so well last season. The club has a good fan base and a good manager in Stuart McCall. He was a big pull in me coming here and I am excited about working with him."
He added: "Motherwell as a whole has a good reputation, I have heard nothing but good things and I just can't wait to get the season started now."
Vigurs started his professional career with Elgin before moving to Inverness Caledonian Thistle in 2006. He was with Caley for three years before moving across the Kessock Bridge to join the Staggies in 2009.
He enjoyed a productive stint at the Global Energy Stadium, including a Scottish Cup runners-up medal in 2010 and an First Division championship in 2011/12.
Steelmen boss Stuart McCall has managed to fend off competition for his signature from both north and south of the border as the club continues its summer recruitment plan.
The Motherwell manager has already lured Lawson to Lanarkshire from County as he prepares for the Europa League qualifiers next month. And more new faces are expected after the departures of Nicky Law, Darren Randolph, Steven Saunders, Omar Daley, Steven Hetherington, Josh Watt, Nicky Devlin, Kallum Higginbotham, Chris Humphrey and Henrik Ojamaa.
Well chief executive Leeann Dempster added: "We are delighted we have managed to recruit both Iain and Paul for next season.
"Obviously this is a summer of change at Fir Park but with that comes a lot of excitement and Iain's signing typifies the calibre of player we are looking to bring in for our European and domestic campaign. In partnership with the manager, we are working through a list of targets and we are hoping to announce more new signings soon."