The Cheshire zoo, which is home to more than 11,000 animals, was also rated the second best in Europe.
The 2014 Travellers' Choice Awards feature categories including landmarks, hotels and beaches.
Jamie Christon, managing director of Chester Zoo said: "We're absolutely delighted."
He said: "Our sincere thanks go to every single person who has come here because, as a registered charity, their visits help us to carry out all of our vital conservation work - saving species from extinction both here in the UK and overseas."
According to the poll, the UK has the most award-winning zoos of all European countries.
Among the other venues winning plaudits were Colchester Zoo, Essex; Paignton Zoo Environmental Park, Devon; Blackpool Zoo, Lancashire; Cotswold Wildlife Park and Gardens, Burford, Oxfordshire; Knowsley Safari Park, Merseyside; and South Lakes Safari Zoo, Dalton-in-Furness, Cumbria.
Chester Zoo has been named the best zoo in the UK and the sixth best in the world by users of travel website TripAdvisor.