Here we present the results of an intensive physical oceanography study conducted in the Reloncavi fjord (41.5º S, 72.5º W) which was focused on the sub-inertial timescale. The along-fjord currents presented 3-day oscillations which were consistent with the natural internal period of oscillation of the fjord basin (internal seiche). This oscillation could explain more than 44 % of the 3-day variability and contributed with kinetic energy levels as large as the tidal currents.