Different lines of evidence suggest that the occurrence and extent of local adaptation in high gene flow marine environments – even in mobile and long-lived vertebrates with complex life cycles – may be more widespread than earlier thought. We conducted a common garden experiment to test for local adaptation to salinity in Baltic Sea sticklebacks (Gasterosteus aculeatus). Fish from three different native salinity regimes (high, mid and low) were subjected to three salinity treatments (high, mid and low) in a full-factorial experimental design. Irrespective of their origin, fish subjected to low (and mid) salinity treatments exhibited higher juvenile survival, grew to largest sizes and were in better condition than fish subjected to the high salinity treatment. However, a significant interaction between native and treatment salinities – resulting mainly from the poor performance of fish native to low salinity in the high salinity treatment – provided clear cut evidence for adaptation to local variation in salinity. Additional support for this inference was provided by the fact that the results concur with an earlier demonstration of significant differentiation in a number of genes with osmoregulatory functions across the same populations and that the population-specific responses to salinity treatments exceeded that to be expected by random genetic drift.