Waterfalls are inspiring landforms that set the pace of landscape evolution as a result of bedrock incision1-3. They communicate changes in sea level or tectonic uplift throughout landscapes2,4 or stall river incision, disconnecting landscapes from downstream perturbations3,5. Here we use a flume experiment with constant water discharge and sediment feed to show that waterfalls can form from a planar, homogeneous bedrock bed in the absence of external perturbations. In our experiment, instabilities between flow hydraulics, sediment transport and bedrock erosion lead to undulating bedforms, which grow to become waterfalls. We propose that it is plausible that the origin of some waterfalls in natural systems can be attributed to this intrinsic formation process and we suggest that investigations to distinguish self-formed from externally forced waterfalls may help to improve the reconstruction of Earth history from landscapes.