The model, first discovered on ITV1 daytime magazine show This Morning in 2001, will work to highlight the animal charity's work.
"It's a real honour to be invited to be Battersea's first ambassador," Gandy said. "The work they do to find homes for thousands of dogs and cats every year is amazing and I hope to help make a real difference.
"What makes Battersea so special is it cares for every type of dog and tries to help them get a second chance in a loving new home."
The 32-year-old, who recently split from The Saturdays' star Mollie King, continued: "From visiting the home, I've found that these dogs, many of them Staffordshire Bull Terriers, are desperate to get close to you for some love and attention.
"The affection they show you is astounding. I hope my involvement will help explain that and make people think of choosing a rescue dog or cat from Battersea for their next pet."
Claire Horton, chief executive of Battersea, added: "We are immensely grateful to David for being such a supporter of our work and agreeing to be our first ambassador.
"David has so many followers and we know his support will really go a long way in helping change attitudes, especially towards more responsible pet ownership. We really couldn't ask for a better champion."
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