Delfino's small-screen credits include "Roswell," "Quarterlife" and "State of Georgia."
Rainn Wilson has played Dwight on "The Office" for eight seasons and is expected to return for the upcoming ninth.
EW.com said former "Office" show-runner Paul Lieberstein is writing the script for the spinoff pilot, "The Farm," which follows Dwight as he runs his family beet farm and bed and breakfast.
Delfino will play Fannie Schrute, Dwight's "pseudo-intellectual lefty" younger sister, who has been living in Boston, the entertainment news Web site said.
The character has not been featured on "The Office" before.