Police said the attack at the zoo in Novosibirsk occurred in an area of the enclosure the woman cleaning, RIA Novosti reported.
A partition between the animal's open-air enclosure and their indoor pen had been accidentally unlocked and the women entered an area occupied by a 5-year-old female jaguar and her 7-month-old cub, zoo director Rostislav Shilo said.
The 48-year-old woman died instantly when the jaguar broke her back, Shilo said.
The woman had worked at the zoo for two years.
The part of the zoo where the attack occurred has been closed.