The turbulent boundary layer under a free-stream whose velocity varies sinusoidally in time around a zero mean is calculated using a Reynolds stress model. Direct simulation (DS) data by Spalart and Baldwin are used to assess the performance of the turbulence model. The model is a full second-moment closure applicable right up to a wall, which was previously shown by the author to reproduce various turbulent flows. Extensive comparison between the DS data and the predictions is made on variations of the wall shear stress, the mean velocity profiles, and the distributions of all the Reynolds stress components. The agreement is generally excellent. In particular, a laminarization during the acceleration phase and a succeeding retransition are reproduced well by the model calculation.