Zuri, 12, attacked 10-year-old Nyack and staff at the Indianapolis Zoo could not separate the pair, reports the BBC. He died of suffocation.
The pair had lived in the same enclosure for eight years and had three cubs together in 2015.
In a Facebook post, the zoo said staff would conduct a thorough review to try to find out what happened. "Nyack was a magnificent lion and he will be greatly missed."
The staff was alerted by "an unusual amount of roaring" coming from the lion pen, the BBC said.
Zuri was holding Nyack by the neck, and despite efforts to separate the pair the lioness continued to hold her partner until he had stopped moving.
There had never been no noticeable aggression between the pair, the zoo said.
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