During 2002−2015 we examined the causes of mortality in beachcast northern sea otters (Enhydra lutris kenyoni) collected from the coasts of Washington and Oregon states. A total of 333 moribund or dead sea otters were reported through opportunistic observations and 93 otter carcasses were collected for necropsy. Retrieved carcasses received a full diagnostic evaluation to determine cause of death when carcass condition permitted. Otter stranding reports were received from the entire Washington coast and northern Oregon coast, but carcasses located near areas populated by humans were most often in suitable condition for cause of death determination. Data in this release include basic demographic information on the carcasses retrieved and the cause of death diagnosed for each individual, annual counts of stranding reports and carcass retrievals, and approximate geographic locations where beachcast otters were observed or collected.