Human-derived reductions to seawater pH (ocean acidification), challenge growth and survival of a diversity of marine invertebrates, especially those that calcify. Here we test the role of the outer organic layer of mussel shells (the periostracum) in preventing shell loss in adult California mussels (Mytilus californianus) exposed to reduced seawater pH. We observed that the periostracum inhibits dissolution of mussel shell at each of three levels of pH (7.7, 7.5, and 7.4 on the total scale), with dissolution rates declining by more than 80% if the periostracum remains intact. We also describe how mussels living higher on the shore and at sites exposed to direct sun tend to exhibit decreased cover of periostracum, suggesting accumulated damage from heat and desiccation at low tide could influence the accompanying ability of mussels to cope with pH stress when immersed. Finally, given periostracum is not living tissue and can be eroded over time, for example by sediment scour, we explore how abrasive removal of the periostracum by sands of differing coarseness affects shell loss, documenting a positive relationship between sand grain size and dissolution rate. These findings highlight the importance of considering protective measures marine organisms may employ to confront global change, and how those mechanisms operate in the face of multiple axes of environmental stress.