Dissimilar flexibility of the soil-foundation system among different legs of the lattice tower structures and possible differential settlement of each foundation could alter the structural tower’s overall behavior, failure mode, and final load-carrying capacity. Besides, lattice tower structures can experience bolt slippage and member buckling, and are subjected to material nonlinearity. This study aims to investigate the effects of differences in soil-foundation stiffness of lattice tower structures while other specific behaviors of such structures, including material nonlinearity, bolt slippage, the eccentricity of connections, and bolted joints stiffness properties, have come into account. To do so, performing incremental nonlinear analysis, advanced numerical finite element models of a complete lattice tower structure are developed, and different probable scenarios for foundation flexibility are defined. Foundation stiffness configurations that produce the worst effect on the tower strength are identified. The proposed model could be further developed to study the allowable limits of differential settlement for lattice tower design.