Mother replaces J Walter Thompson London, which split with the retailer following an eight-year partnership.
The agency pitched against Karmarama, Portas and Mcgarrybowen in the review, which was managed by AAR.
Starting immediately, Mother will be responsible for developing Debenhams’ brand identity and expressing its new strategy that was put in place by chief executive Sergio Bucher earlier this year.
With a focus on customer-centric communications, the brand aims to become a destination for "social shopping" and make shopping "confidence-boosting, sociable and fun".
Richard Cristofoli, the managing director of beauty and marketing at Debenhams, said: "Throughout the pitch process, Mother consistently demonstrated that they understand us, our customer and our brand, as well as the challenges in hand, and could bring their creative thinking in a way which would make a difference. We are excited to work with them as we move forward with our Debenhams Redesigned strategy in 2018."
The media planning and buying account, held by Carat, was not affected by the review.
JWT London’s last Christmas campaign for Debenhams retold the classic fairy tale Cinderella and featured Ewan McGregor.