Estuary habitats may provide key rearing and growth opportunities to juvenile Pacific salmon as they migrate from natal freshwaters to the ocean. Yet current understanding the contribution of the estuarine life stage to individual fitness and survival across the salmon life-cycle remains limited. For example, estuary residence may confer a growth benefit to migrating juvenile salmon, which in turn could reduce size-selective mortality risk in the ocean. Here we consider how juvenile salmon use estuary habitats for rearing and growth in the course of their seaward migration. We performed a multi-year mark-recapture study of coho salmon in the relatively pristine Koeye River and its estuary, located on the remote Central Coast of British Columbia. Most recaptured juveniles attained additional growth in the estuary before entering the ocean, with small juveniles benefiting most from this estuary rearing period. While large juveniles transited the estuary in a just a few days, their smaller conspecifics spent up to 40 days residing and feeding in the estuary before marine entry, resulting in up to 25% increase in body size. Consequently, the outmigrating cohort entered the ocean both larger and less variable in size than it was at freshwater exit. Analysis of returning spawners indicated that larger juveniles tended to have higher ocean survival rates than smaller individuals. Together, these results suggest that growth achieved during estuary stopover could mitigate freshwater carryover effects, with the potential to boost watershed-level productive capacity and buffer size-selective survival in a changing ocean.