Rose left the TV show following the first season after she sustained an injury during a stunt, suffering two slipped discs which were close to severing her spinal cord and threatened to leave her paralysed.
Bruce Wayne’s cousin Kate Kane, who until recently was Batwoman, was presumed dead at beginning of the latest season, when she was involved in a plane crash.
However, and spoilers are coming, on Sunday night’s episode, she was revealed to be alive, but unrecognisable, and shown on screen to be covered in bandages while being held hostage.
The show, which goes out on The CW in the US and E4 in the UK, will now move forward with the “altered version” of Kane’s character, and use her mystery reappearance as part of the storyline.
Writing on Instagram, Day said: “Super excited to finally announce I’m joining the cast of Batwoman.
“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. Everyone’s made me feel so welcome and it’s amazing being back home with my DC family.”
Kate Kane, however, is no longer the alter-ego of Batwoman.
Instead, her new non-hero form is known as Ryan Wilder, played by Javicia Leslie – the first black actor to portray Batwoman.
The show is seen as a trailblazer in LGBTQ+ visibility, and is the first live-action TV series to focus on a lesbian superhero.
Rose faced an intense backlash after the show’s first season went out, with some claiming that she was not was not “gay enough” to play the role, with others saying that she should have allowed a Jewish actress to play Kate Kane, who is written in comics to be of Jewish descent.
It led to Rose leaving Twitter, and disabling public comments on her Instagram account.