Jaina will be taking on the female lead role in the 10- episode spin-off, reported Deadline.
Director/EP Paris Barclay took to her Twitter account to give a shout-out to the 30-year-old actress. "She's our star!" she wrote.
The yet-untitled spin-off will follow the group of firefighters in a Seattle firehouse.
The first episode of the series is set to be launched as a planted spin-off and air in one of "Grey's Anatomy" episodes this fall.
The lead characters will be introduced in the episode before the spin-off launches in midseason.