Armitage, who made five appearances for England and played more than 300 professional matches in England and France, winning three European Cup championships, will join the Legion for pre-season training in December and said he hoped to add another trophy to his resume.
After eight seasons in France, the last three with Pau, Armitage was out of contract and looking for a new club.
Rather than move down a flight, the 33-year-old flanker decided to try his luck in the United States.
“I’m excited to play in the fastest-growing rugby competition in the world,” said Armitage in a statement. “San Diego has a very strong team and fantastic culture and I look forward to becoming part of the Legion with hopes of delivering America’s Finest City the MLR title in 2020.”
San Diego came close to lifting the MLR silverware this season, losing 26-23 to Seattle Seawolves in June's championship game.
“Steffon is a world-class player who has proven himself to be one of the greats in the European club game," said Legion head coach Rob Hoadley. "His talent and leadership will be a perfect addition to the Legion organization.”
More and more aging top players are looking at the new North American league in much the same way that David Beckham, Wayne Rooney and other top Europeans in the later stages of their careers have looked to Major League Soccer.
Rugby United New York earlier announced the signing of French international Mathieu Bastareaud who will join former England fullback Ben Foden, capped 34 times for his country, in the Big Apple next season. (Reporting by Steve Keating in Toronto; Editing by Clare Fallon)