A statement from police in the county of Cambridgeshire said they arrived at the scene of the "serious incident" at Hamerton Zoo Park at around 11.15 am (1015 GMT).
"A tiger had entered an enclosure with a keeper. Sadly the female zookeeper died at the scene.
"At no time did the animal escape from the enclosure. The incident is not believed to be suspicious," the Cambridgeshire Constabulary said on its Facebook page, without giving further details.
In a statement posted on its Facebook page, the Hamerton Zoo Park said the incident "appears to have been a freak accident" and that a "full investigation is currently under way and we hope that more details can be announced as soon as we are able."
"All our thoughts and sympathies are with our colleagues' friends and families at this dreadful time," the zoo said.
No other animals escaped their enclosures and public safety was never a concern, the zoo added.
According to the zoo's website, the 25-acre park is home to Malaysian and Bengal tigers.
The zoo will be closed on Tuesday.
