Project Survival's Cat Haven announced the passing of Titan on its Facebook page.
It shared a video of the 8-year-old lion on his last birthday this past September.
A little more than a week ago, Cat Haven staff said Titan was undergoing medical tests because he wasn't eating well.
A cause of death is not yet known and a necropsy will be done to learn more.
Animal experts say some lions can live as long as 20 years.
Titan was one of two lions at the sanctuary.
Staff members say his female companion, Pely is rather distraught that her partner is gone.