The aim of this study is to clarify the mechanical behavior of the joint parts in a steel pipe sheet pile (SPSP) foundation under horizontal loading. The foundation structure of SPSPs is constructed by coupling the joints of each SPSP in the ground with the injection of mortar into the joints. For this purpose, the mechanical behavior of the SPSP foundation should be discussed with a clear understanding of the mechanical behavior of the SPSP joints. In the design criteria under lateral loading, however, only the vertical shearing of the joint parts is considered as critical. Accordingly, the precise understanding of the lateral loading capacity of the SPSP foundation was investigated in this study through model experiments and a reproduction analysis by the 3D finite element method. Simply stated, from the lateral loading tests on the SPSP models and their reproduction analysis by a 3D FEM, it was found that the lateral loading causes vertical shear displacements of the joints of the piles arranged in parallel to the loading direction and almost no displacements of the joints of the piles arranged perpendicularly to the loading direction.