Elevated water tanks are critical lifeline structures whose seismic performance depends strongly on their dynamic characteristics, particularly the fundamental period and damping ratio. Numerical models often rely on simplifications that may not capture real behavior. This study presents an experimental forced vibration test on a 600 m³ reinforced concrete elevated water tank in Rudsar, Iran, to determine its fundamental period and damping ratio under different water levels. A shaker (Pooya5000) was installed on the roof, and harmonic excitation was applied at frequencies up to 8 Hz. Accelerations were recorded, processed using SeismoSignal (bandpass filter 0.5–7 Hz), and compared with finite element (SAP2000) and analytical (Housner-type) models. Results show that the experimentally obtained fundamental periods (0.525–0.554 s for water depths 2.35–3.63 m) are significantly lower than FEM (0.74–0.77 s) and analytical (0.70–0.73 s) predictions. The average damping ratio from free vibration decay was only 0.89%, attributed to the very low excitation amplitude (elastic range). This study highlights the necessity of experimental validation for dynamic analysis and suggests that current code-based period formulas may overestimate flexibility in such structures.