Deadline reports that the former Lie To Me star will play CIA agent Rex Matheson in the new ten-part run, subtitled 'The New World'.
The character was previously described as a Harvard-educated "golden boy" who is "dangerous, dazzling [and] reckless".
Phifer was a series regular on ER from 2002 to 2008 and also played a recurring role on Curb Your Enthusiasm. His film roles include 8 Mile and the 2004 Dawn of the Dead remake.
It was reported last month that Crash actress Arlene Tur had also been cast in the sci-fi drama as surgeon Vera Juarez.
Torchwood: The New World will air on BBC One in the UK and Starz in the US.