Bats are a concern for proposed offshore wind energy development due to significant bat mortality occurring on terrestrial wind farms and documented observations of migratory bats offshore. This concern is further amplified by our limited understanding of bat populations, species distributions, migration behavior, and habitat use, particularly in the offshore environment. We sought to establish baseline information about bat migration along the California coast and offshore rocks and islands, creating the first systematic study on the West Coast. We established 20 sites, 10 coastal onshore sites and 10 sites on offshore rocks and islands spanning nearly the entire California coastline where we deployed acoustic bat detectors. We processed, auto-classified, and manually vetted the data to confirm species presence and activity. Both migratory and resident species are active in coastal and offshore environments throughout the year, with peaks in activity during spring and fall. Activity at offshore and island sites is significantly lower than coastal sites and northern sites have substantially higher activity than southern sites for most seasons and species. This research demonstrates coastal and offshore environments are frequently used by bats, which has implications for management of offshore wind energy development.