The effect of slow rotation on the propagation of solitary internal waves in shallow fluids is studied. The Kadomtsev–Petviashvili (KP) equation, appropriately modified to account for Coriolis effects, is solved numerically for two‐layer flow in a rotating channel of finite width. In the presence of rotation, an initially straight‐crested soliton evolves to a three‐dimensional disturbance, which, to a reasonable approximation, has a sech2profile along the channel but decays slowly as it propagates. Furthermore, the wave amplitude varies exponentially across the channel and the wave crest is curved backward. These trends are in agreement with recent laboratory experiments.