The design of dam structures requires advanced analysis techniques due to the dam body's complicated interaction with its surroundings. Consequently, most of the time, it is a must to perform response history analysis during the design and assessment of dam structures. The target fundamental vibration period is necessary to generate ground motion histories compatible with the site-specific response spectrum. In this study, an equation was developed to predict the fundamental vibration period of concrete gravity dams based on an extensive database. First, a wide range of parameters to describe gravity dams' three-dimensional (3D) geometry and their valleys are determined. A total of 19,440 different 3D numerical models with fluid and solid elements were formed, and the fundamental vibration period values were determined to create the necessary database. Then, multiple nonlinear regression analysis was conducted to propose an equation to predict the fundamental vibration period. The proposed equation could be used to accurately estimate the required fundamental vibration period during the preliminary design stage or the implementation of simplified procedures or during the spectrum matching operations.