AbstractShock waves in collisionless plasmas are among the most efficient particle accelerators in space. Shock reformation is a process important to plasma heating and acceleration, but direct observations of reformation at quasi‐parallel shocks have been lacking. Here, we investigate Earth's quasi‐parallel bow shock with observations by the four Magnetospheric Multiscale spacecraft. The multi‐spacecraft observations provide evidence of short large‐amplitude magnetic structures (SLAMS) causing reformation of the quasi‐parallel shock. We perform an ion‐kinetic Vlasiator simulation of the bow shock and show that SLAMS reforming the bow shock recreates the multi‐spacecraft measurements. This provides a method for identifying shock reformation in the future.