Migrations allow fish to exploit resources that vary across time and space to increase lifetime fitness. For anadromous salmonids, migrating to sea enables rapid growth and is often more advantageous for females, whose reproductive success is linked to body size. Female smolts may therefore show different migration behaviour and survival than males. To test for sex-specific influences on migration behaviour and survival, we tagged Atlantic salmon smolts with acoustic transmitters in the Lower River Bann (n = 100) and River Bush (n = 120) in Northern Ireland, in 2024, and determined sex by genetic analysis. Tagged smolts were tracked across riverine, coastal and offshore acoustic arrays. Although there were more females in both rivers, the sex ratio remained unchanged over the capture period, and there were no sex differences in diel patterns, migration time, or survival during riverine and coastal migration. Females showed a higher probability of daytime arrival and were detected across a broader portion of the offshore array than males, which may suggest that females initiate foraging earlier during marine migration. Tidal cycles and water temperature influenced migration in both rivers, but effects depended on local conditions. These findings provide the first comparative field test of sex differences in salmon smolt migration, indicating that although females more frequently adopt anadromy, both sexes have similar behaviour and survival during the freshwater-marine transition. As female survival has a disproportionately large effect on population productivity, understanding sex-specific behaviour and survival is essential for the conservation and management of salmonids.