The former Eternal singer will join Nigel Lythgoe and Arlene Phillips on the panel for the dance contest, which launches tomorrow night on BBC One.
"I'm so excited about this opportunity," said Redknapp. "I went through similar dance training as a lot of the kids competing, so I know where they're coming from. The opportunity this show offers them is phenomenal, and I can't wait to be part of the live shows."
Lythgoe, who is also executive producer on the series, added: "Louise joined us as a guest judge during the Manchester leg of the auditions and we loved her - she brought something different to the panel and she had a great affinity with the kids up there dancing for their lives. When it came to thinking about the live shows, we kept coming back to Louise. She is perfect for the job."
Redknapp was recently unveiled as the new host of Sunday morning show Something For The Weekend.
> Click here to see Digital Spy audition for So You Think You Can Dance