To analyze the influence of an adjacent foundation on ground stiffness, a new concept of ground stiffness influence coefficient is introduced, which is defined as the ratio of the ground stiffness with the adjacent foundation to that without. Based on the Mindlin solution, an analytical expression of the influence coefficient is derived from a system of two rigid, surface, rectangular foundations under three-dimensional elastic half space. Through calculating the influence coefficient, we analyze the influences of clear distance, side-length ratio of foundations and Poisson’s ratio of the ground soil on ground stiffness when the adjacent foundation exists. It is shown that: (1) The ground stiffness can be improved when the effect of the adjacent foundation is taken into account. (2) The influence of the adjacent foundation on ground stiffness shows a positive correlation with the side-length ratio and the Poisson’s ratio, and a negative correlation with the clear distance. (3) The influence is more significant in clay ground than in other kinds of ground, such as silt ground. (4) It is not necessary to consider the influence of adjacent foundation when the clear distance is larger than 15 times the side length of the foundation or the side-length ratio is less than 0.1. In other cases, however, we should decide whether or not it is necessary to consider the influence of both the clear distance and the side-length ratio.