The Ducks announced their entry-level deal with the defenceman Tuesday.
Theodore was the 26th overall pick in the NHL draft last June.
He has one assist in his first three preseason games with Anaheim during training camp. He played more than 22 minutes Sunday against Colorado, leading all Ducks in ice time.
Theodore scored 50 points in 71 games with the WHL's Seattle Thunderbirds last season. He also played for Canada at the under-18 world championships.