Abstract Direct measurements of return‐stroke current propagation speed inside lightning channels remain extremely challenging, so the propagation speed of return‐stroke luminosity is widely used as a proxy. We present the first direct measurement of return‐stroke current propagation speed along a natural lightning channel, using synchronized lightning current, electromagnetic field and optical observations. The return‐stroke current propagated downward along an unusually long preceding upward connecting leader (UCL) channel before reaching the top of Canton Tower. The average three‐dimensional (3D) downward propagation speed of the return‐stroke current wave is estimated to be approximately 2.4 ± 0.4 × 108 m/s across a roughly 870 m propagation path. The rising edge of the initial peak of return‐stroke current measured at the channel base is morphologically similar to that of the initial electromagnetic field peak at return‐stroke initiation.