In orthogonal frequency-division multiplexing (OFDM) systems, the capability to support high mobility is greatly limited by the intercarrier interference (ICI) caused by time-frequency selective fading channels. This paper presents a precoded OFDM system for ICI mitigation. A precoder is introduced to relieve the ICI by transmitting N-point composite information symbols at twice the subcarrier interval. A Hadamard-matrix-like pilot pattern is used to recover the composite information symbols in a postprocessor at the receiver. Simulations show that, compared to the conventional self-cancellation scheme, this scheme gives much better signal-to-interference-noise ratio performance with much less overhead. Furthermore, the scheme can support twice the vehicle speed in time-frequency selective fading channels than the standard OFDM systems without ICI mitigation.