Autosomal recessive mutations in Anoctamin 5 (), a member of the transmembrane 16 (TMEM16) family of Ca-activated ion channels and phospholipid scramblases, cause adult-onset muscular dystrophies (limb girdle muscular dystrophy 2L (LGMD2L) and Miyoshi Muscular Dystrophy (MMD3). However, the molecular role of ANO5 is unclear and knockout mouse models show conflicting requirements of ANO5 in muscle. To study the role of ANO5 in human muscle cells we generated a myoblast line from a MMD3-patient carrying the c.2272C>T mutation, which we find causes the mutant protein to be degraded. The patient myoblasts exhibit normal myogenesis, but are compromised in their plasma membrane repair (PMR) ability. The repair deficit is linked to the poor ability of the endoplasmic reticulum (ER) to clear cytosolic Ca increase caused by focal plasma membrane injury. Expression of wild-type ANO5 or pharmacological prevention of injury-triggered cytosolic Ca overload enable injured patient muscle cells to repair. A homology model of ANO5 shows that several of the known LGMD2L/MMD3 patient mutations line the transmembrane region of the protein implicated in its channel activity. These results point to a role of cytosolic Ca homeostasis in PMR, indicate a role for ANO5 in ER-mediated cytosolic Ca uptake and identify normalization of cytosolic Ca homeostasis as a potential therapeutic approach to treat muscular dystrophies caused by ANO5 deficit.