The actress revealed on Twitter that she will star in the sixth episode, titled 'Harvest'.
She wrote: "For those who were wondering: Smallville ep. 'Harvest' is the one I'm in."
Executive producer Al Septien also revealed details of Doig's role on the show on his own Twitter feed.
"Lexa's awesome," he wrote. "She plays Dr. Christina Lamell, [who is] working with Tess (played by Cassidy Freeman)."
Doig starred in Andromeda from 2000 to 2005 and recently played a recurring role on ABC's V.
The final season of Smallville begins September 24 on The CW.