Day shared the exciting casting news to her Instagram on Sunday.
"Super excited to finally announce I’m joining the cast of Batwoman," Day wrote on Instagram, alongside a screenshot of a report announcing the casting news. "I’m sure you can imagine how much this means to me and how incredible it’s been working on the show so far."
"It’s a huge responsibility stepping into the shoes of a character that’s already been established and it’s a journey I’m looking forward to being a part of and hoping you guys do too," Day wrote in her caption. "Everyone’s made me feel so welcome and it’s amazing being back home with my DC family."
During the first season of Batwoman, Kate Kane -- the cousin of Bruce Wayne -- takes on the role of Batwoman to serve as a vigilante in Gotham City.
After the first season, Rose opted to leave the series -- partly due to a back injury she suffered on set during production -- and the show wrote Kate Kane off, saying her character disappeared and was presumed dead after a plane crash.
In season two, actress Javicia Leslie took on the title role, playing Ryan Wilder, a woman who takes on the role of Batwoman after finding the suit in the wreckage of the plane crash. Leslie is the first Black actress to portray the character of Batwoman on screen.
The move to bring back the Kate Kane character -- who will not be taking back the Batwoman mantle -- appears to have been in the works since she was written off in a mysterious way, and it appears that with Day being cast, fans will learn what has happened to her since the crash.
Leslie spoke with ET in January and opened up about getting to take over as the eponymous hero after Rose's departure.
“We worked really hard to come back from what I think people thought would break us," Leslie told ET’s Leanne Aguilera. "I'm really proud of our team for making magic."
Check out the video below to hear more.
Ruby Rose on Fighting Through Her Severe Injury and Javicia Leslie Taking Over ‘Batwoman’ Cowl