The response of a meandering river to lateral tilting of the floodplain that supports the river is studied theoretically. Here, the lateral direction is perpendicular to the overall direction of the river on the floodplain. It is found that for most natural meandering rivers, the river will migrate down the tilt direction to a lower elevation. However, the theory also shows that under some conditions, especially when the Froude number of the flow in the channel is very small, the river migration is biased in the opposite direction. Two major contributions to the response of the river to lateral tilting, identified in the theory, are (1) perturbation of the cross-section-averaged flow velocities and (2) modification of the near-bank flow velocity variation along the channel. The theory is one of the first attempts to develop a quantitative understanding of the effects of floodplain tilting on meandering rivers, and it provides a framework for future studies.