The Gulf Stream is a major conduit of warm surface water from the tropics to the subpolar North Atlantic. Here we observe and simulate a sub-mesoscale (<20 km) mechanism by which the Gulf Stream exchanges water with subpolar water to the north. Along isopycnals, the front has a sharp compensated temperature-salinity contrast, with distinct “mixed” water between the two water masses 2 and 4 km wide. This mixed water does not increase downstream despite substantial energy available for mixing. A series of “streamers” detrain this water at the crest of meanders. Subpolar water replaces the mixed water and resharpens the front. The water mass exchange accounts for a northwards flux of salt of 0.5 − 2.5psum2s−1, (large-scale diffusivity O(100m2s−1)). This is similar to bulk-scale flux estimates of 1.2psum2s−1, and supplies fresher water to the Gulf Stream required for the production of 18-degree subtropical mode water.