A video shows the moment the man is attacked as he can be heard screaming in pain.
Ignoring the animal’s snarling, the keeper attempts to play with and tease the lion as he sticks his fingers in its mouth.
He then sticks his fingers into the cage again and the lion bites down.
The man tries to pull his hand back but the animal has its jaws clamped around his middle finger.
As he tries to wrench his hand away, he braces himself with his foot against the concrete enclosure and pulls away.
However, he left behind his finger in the creature’s mouth as the force of the pull severed it from his hand.
A witness told the Jamaica Observer: “When it happened, I thought it was a joke.
“I didn’t realise the seriousness of it because it’s their job to put on a show.
“Obviously, when he fell on the ground everybody realised that it was serious. Everybody started to panic.”
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“I ran away from the whole thing, because I didn’t like to see blood and it was too graphic for me.
“His facial expression when he was walking is like the pain never kicked in.”
The zoo claimed it was not aware of the incident but it is now being investigated by the Jamaica Society for the Prevention of Cruelty to Animals.
Managing director Pamela Lawson said: “We will be going down there and I will be communicating with the National Environment and Planning Agency who has oversight over Jamaica Zoo.”