In this study, the effect of foundation soil stiffness on the seismic performance of integral bridges (IBs) is investigated. For this purpose, nonlinear structural models of a two-span TB with four different foundation soil stiffness types (loose, medium, medium-dense and dense sands) are built. In the nonlinear structural models, nonlinear soil structure interaction including free-field effects is considered. Then, the nonlinear time history analyses of the TB models are conducted using a set of ground motions with various intensities. The analyses results reveal that foundation soil stiffness has significant effects on the seismic performance of IBs, particularly under large-intensity earthquakes. Stiffer foundation soils are found to produce smaller maximum absolute displacement in the deck of IBs, especially in the case of large-intensity earthquakes. This is mainly due to the larger foundation flexibility in the case of soft soil conditions producing larger deck displacements. For the bearings, however, because of their large flexibility, the difference between the bearing displacements for various foundation soil stiffness conditions is negligible. Furthermore, for IBs built on soft soil conditions, the pier columns and steel H piles are observed to experience less damage (better performance) in the case of a potential earthquake.