United adds depth to its midfield after acquiring the Canadian international, who impressed during preseason.
Porter, 23, has not made an appearance with the MLS version of the Whitecaps, but was a standout in 2011 and 2012 with FC Edmonton of the NASL.
“Kyle is an excellent prospect for our club,” said D.C. United general manager Dave Kasper. “He is an athletic midfielder with great range and the ability to manufacture goals. We look forward to Kyle's future with United.”
Porter earned his first two caps with the Canadian national team in January.