The utility back, the younger brother of former All Black Rodney So'oialo and ex-Harlequins scrum-half Stephen So'oialo, joins from Wellington Norths on a two-year deal.
The 24-year-old was part of Samoa's 2011 World Cup squad and has been included in their party for the upcoming tour to South Africa.
Pat Lam, Connacht's new head coach, said: "James is a superb utility back who can play all positions but half-back.
"He can kick goals from up to 60 metres out and I'm sure with the Galway wind, we will see him kick from even further out.
"To have someone in our squad with so much versatility and who can kick goals from that distance is a real weapon in our armoury."