The Seto Inland Sea is the largest enclosed sea in Japan. Numerous large-scale coastal developments such as land reclarnation, bridge construction, and dredging have greatly changed the basin scale of the Seto Inland Sea since the 1960s. We investigated the interannual tidal variation during the last 40 years and studied the possible effects of modern coastal developments on the tide by using a one-dimensional tidal model. Interannual tidal variation has shown some very significant secular changes. The amplitude of the principal lunar semidiurnal constituent, M-2, in the central portion of the sea increased from 1960 to the late 1980s and since then has been decreasing. In the eastern portion of the sea, on the other hand, the amplitude decreased up to the late 1980s and has since been increasing. The model showed that the secular changes from 1960 to the late 1980s could be attributed to the enhancement of response due to coastal development in the Seto Inland Sea. It further showed that secular changes after the late 1980s could be attributed to the weakening of the response due to coastal developments where the cross-sectional area is minimum in the Seto Inland Sea.