Emilia Clarke and Jason Clarke, who are of no relation to each other, have already been cast as Sarah and John Connor - character names that are already familiar to fans of the franchise that started with the 1984 Arnold Schwarzenegger movie.
Smith will play a new character with a strong connection to John Connor, the studio reported.
Alan Taylor is directing the film from a screenplay by Laeta Kalogridis and Patrick Lussier. The cast also includes Jai Courtney, J.K. Simmons, Dayo Okeniyi, Byung Hun Lee and, of course, Schwarzenegger.
David Ellison and Dana Goldberg of Skydance are producing. Skydance's Paul Schwake, Annapurna Pictures' Megan Ellison, Kalogridis and Lussier are executive producing.
Smith is repped by United Talent Agency and Michael Duff at Troika.
Paramount will distribute the film worldwide on July 1, 2015.
Variety sister site Deadline Hollywood broke the news of the casting.
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