Kildunne - who scored 52 tries for Quins and was named World Player of the Year during her time at the Stoop - moves to the West Country in a major coup for the Bears.
The full-back had previously said in an Instagram post that she was "gutted" to be leaving Quins and that the decision was "not in my control", but she has greeted her next move with enthusiasm after what appeared to be a painful departure from south west London.
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"Bristol Bears feels like the perfect club for me at this stage of my career," Kildunne said.
"The way the team plays is hugely exciting and completely aligned with how I want to express myself on the pitch. The ambition, the style and the belief within the group really stood out whenever I have played against them.
"I've always loved playing in Bristol - both for club and country. The World Cup semi-final at Ashton Gate was an incredible occasion and the fans there are hugely passionate and create an incredible atmosphere.
"Bristol has built a fantastic brand in women's rugby. It's a club that's ambitious, innovative and isn't afraid to do things differently. That's something I really wanted to be a part of."
"Signing Ellie is a huge statement of intent and we're delighted that she'll be a Bristol Bears player as she comes into the peak years of her already incredibly successful career," Bears head coach Scott Lawson added.
"She is not only the World Player of the Year and one of the finest talents on the planet, but also a player who inspires people far beyond rugby. Her ability to excite supporters, influence matches and drive standards makes her a perfect fit for what we're building at Bristol Bears.
"We're thrilled that she has chosen Bristol as the next chapter in her career."
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