Mucolipidosis type IV is a still poorly understood lysosomal storage disease caused by alterations in the mucolipin lysosomal calcium channel. In this study, we generated mucolipin knockout Dictyostelium cells, and observed that lysosome exocytosis was markedly increased in these cells compared to wild-type cells. In addition, mucolipin KO cells were more resistant to calcium deprivation, and the calcium concentration in their secretory lysosomes was decreased, suggesting that mucolipin transfers calcium ions from the cytosol to the lumen of secretory lysosomes. We speculate that mucolipin attenuates the fusogenic effect of local cytosolic calcium increases by dissipating them into the lumen of lysosomal compartments.