The 26-year-old actress, who played Sonia Fowler on the BBC One soap, decided to sign up for the programme after realising that it could help her to lose weight, The Sun claims.
An insider revealed: "Natalie's thrilled. She is a huge fan of the show and it's exactly what she needs to kickstart her career.
"Since leaving EastEnders she's not done a huge amount of acting and this could really boost her profile. Natalie really wants to get fit too."
Cassidy released a workout DVD in 2007 after losing 4st with the help of a personal trainer. However, she later shot back up from a size 8 to 14 and admitted that she regretted the project.
The source added: "She's a big star and people really warm to her. Appearing on Strictly is a dream come true for her."
Strictly Come Dancing returns to BBC One in the autumn.