Entertainment Weekly reports that the former Rome star will play new team leader Ilsa.
The "sophisticated, driven and principled" character is described as a rich widow who initially disapproves of the lethal methods employed by Chance (Mark Valley).
Ilsa will be "clearly in charge" of the team and "doesn’t take any nonsense from anyone".
Varma previously played the traitorous Suzie Costello in the first series of Torchwood. She has also made a guest appearance on Bones and recently starred as the ex-wife of the title character in BBC crime drama Luther.
Human Target will return to Fox for a second season of 13 episodes on September 24.