[en] A scheme for producing nearly single-cycle relativistic laser pulses is proposed. When a laser pulse interacts with an overdense thin foil, because of self-consistent nonlinear modulation, the latter will be more transparent to the more intense part of the laser, so that a transmitted pulse can be much shorter than the incident pulse. Using two-dimensional particle-in-cell simulation and analytical modeling, it is found that a transmitted pulse of duration 4 fs and peak intensity 3x10²⁰ W/cm² can be generated from a circularly polarized laser pulse. The intensity of the resulting pulse is only limited by that of the incident pulse, since this scheme involves only laser-plasma interaction.