In this paper, an equivalent circuit model for tunable square slot frequency selective surfaces (FSS) is proposed. Tuning is achieved with the inclusion of a discrete capacitance element, such as a capacitor or varicap, at key points within the FSS unit cell. The equivalent circuit model takes into account the FSS physical attributes, i.e. the unit cell dimensions and spacing. The proposed model is intended for a good first design approach, eliminating the need for extensive parametric electromagnetic simulations. The performance of the proposed equivalent circuit model was evaluated against results obtained from appropriate electromagnetic simulations, based on the root-mean-square error (RMSE) between predicted and measured resonant frequencies of the FSS. Results demonstrate the validity of the proposed equivalent circuit model, in which good estimations of the frequency response of FSS structures were obtained. A RMSE value of 190 MHz was extracted from the complete parametric simulation set. The model was further validated against measurements performed on a tunable active FSS prototype inside an anechoic chamber. Tuning of the FSS was achieved by means of bias voltage to modify the varicaps capacitance value. A relatively good agreement was verified between equivalent circuit model, electromagnetic simulations and measurements. Finally, this work is sought to be a useful contribution to the literature on this topic, by improving on the elementary models of such FSS structures and be useful as a good and fast first approach to determine approximate FSS unit cell dimensions.