The 30-year-old actress, will be back as a series regular in season six and will portray the role of Black Siren, reported Variety.
Cassidy will first appear in the final two episodes of the CW show in the current fifth season.
She was an original cast member of "Arrow" where she essayed the role of Laurel Lance/Black Canary in the first four seasons, before she was killed off in the midst of season four.
After leaving the series, Cassidy signed a deal with the network where she would appear across the DC universe portfolio of shows in a guest capacity, which resulted in the arrival of Black Siren.