Bank worker Rick, 32, paid £2 for the fish from his local supermarket in Belfast and took it home for tea for himself.
He was cleaning the fish and shook it by the tail when the pale-coloured louse popped out of the fish's mouth.
Mr Beattie, from Knocknagoney, Belfast, said: "It was like something from a horror film. I've contacted Tesco and they've apologised and promised me a refund.
"I'm sure other Tesco customers would be interested."
The tongue-eating louse is a common parasite in fish. The creepy creature enters through the gills before attaching itself to the fish's tongue.
In a final gruesome act the symbiotic parasite - Latin name Cymothoa exigua - destroys the fish's tongue and then attaches itself to the stub of what was once its tongue and becomes the fish's new tongue.
Experts believe they are not harmful to humans unless picked up alive, in which case they can bite.
A spokesman for Tesco said: "We have very high standards for food quality and both we and our suppliers have robust checks in place to ensure that our food meets those standards.
"We have asked Mr Beattie to return the product, so we can begin an urgent and thorough investigation with the supplier.
"Our customer service team is in touch with Mr Beattie so we can keep him updated on the investigation."