A three-dimensional sigma coordinate free surface model is used to investigate the semidiurnal tidal exchange through the Strait of Gibraltar. The model makes use of a coastal-following, curvilinear orthogonal grid that includes the Gulf of Cadiz and the Alboran Sea, with very high resolution in the strait (<500 m). A lock-exchange initial condition is used: the western part of the model domain is filled with Atlantic water, whereas the eastern part is filled with Mediterranean water. The model is forced at the open boundaries through the specification of the semidiurnal (M2 and S2) tidal surface elevation. The model is run over a spring neap cycle (fortnightly period), and the results are compared with available observed data. Simulated cotidal maps of the M2 and S2 tidal elevation components are in quantitative and qualitative good agreement with observed data as well as with the simulated major and minor axis of tidal ellipse. The model reproduces the generation and the subsequent propagation of internal bores both eastward and westward, showing that they are always generated during the fortnightly period. However, the principal aim of this work is to quantify the effects of tidal forcing on mean quantities, entrainment, and transport of Atlantic and Mediterranean water along the strait. Model results reveal that the contribution of the semidiurnal tidal component (M2) to the transport is relevant over Camarinal Sill, whereas it is negligible at the eastern end of the strait. Model results indicate, also, that the effect of the semidiurnal tide is to increment the mean transport by about 30% both for the inflow and the outflow.