The afterglows to gamma-ray bursts (GRBs) are due to synchrotron emission from shocks generated as an ultra-relativistic outflow decelerates. A forward and a reverse shock will form, however, where emission from the forward shock is well studied as a potential counterpart to gravitational wave detected neutron star mergers the reverse shock has been neglected. Here, we show how the reverse shock contributes to the afterglow from an off-axis and structured outflow. The reverse shock will be observable as a brightening feature in the rising afterglow at radio frequencies for bursts at 100Mpc and systems inclined 30 degrees (or ~5 times the core opening angle for θ=6 degrees) at ~1−10 days post-merger. For structured outflows, enhancement of the reverse shock by a strong magnetic field within the outflow is required for the emission to dominate the afterglow at early times. Early radio imaging of a reverse shock could reveal the presence of a strong magnetic field associated with the merger remnant.