He drowned after a strong current swept him out to sea as he swam with team-mates during a training camp on the Caribbean island of St Kitts.
Barnaby was sixth in his heat at London 2012 and reached the semi-finals at last year's Glasgow Commonwealth Games.
Athletics Canada chief executive Rob Guy called Barnaby "an outstanding athlete and an even better young man".
Canadian Olympic 400m runner Daundre Barnaby has died in a swimming accident, aged 24.