Knowsley Safari Park confirmed 18-year-old Kenya was attacked by females in the pride.
Despite the efforts of keepers to separate them, he sustained multiple injuries and was put down.
The park said it was "the most necessary and humane course of action" to avoid any further stress and suffering to Kenya.
In a statement, the park said the attack indicated his place as pride male was "no longer secure".
The statement said it was "natural behaviour" for a pride when the male is "no longer able to maintain the grace of the females".
Although there was no prior indication of an attack, staff were all prepared for when the "unpleasant event should arrive".
They added that no members of the public were in their vehicles at the time of the attack.
In the wild, male lions are in their prime between five and 10. Kenya, at 18, was "an old man".
He arrived at Knowsley from Longleat in 2003.
An elderly lion who was attacked by lionesses at a Merseyside safari park has died as a result of his injuries.