Rated as the fourth best North American defenseman by the NHL Central Scouting and despite being one of the youngest eligible in age, Siemens was selected 11th overall in the 2011 NHL Entry Draft by the Colorado Avalanche, with the pick acquired from the St Louis Blues along with Erik Johnson and Jay McClement in exchange for a second round pick, Chris Stewart and Kevin Shattenkirk. On July 28, 2011 the Colorado Avalanche signed Siemens to a three-year deal.