The accident, at Chipperfields Farm, Chipping Norton, Oxfordshire, was the second tragedy involving a tiger to affect the family in a matter of weeks. It is believed the animal swallowed the man's limb after severing it below the elbow.
Nigel Wesson, 32, who had been working at the farm for just five weeks, was taken straight into emergency surgery at John Radcliffe Hospital, Oxford. "He was stabilised and is in theatre. It is impossible to say how long surgery will take because we do not know the extent of his injuries," said a spokeswoman.
Chipperfield Enterprises said the four-year-old tiger had been "bedded for the night as normal".
Early last month, Richard Chipperfield, 24, was seriously injured when a tiger he had raised as a cub clamped its jaws around his head. His brother Graham, 28, was questioned by police after he killed the 21-stone animal, which attacked Richard in Florida.
In yesterday's tragedy the keeper was in the process of trying to move the tiger after feeding. He was outside the cage but put his hand inside to move the partition open.