<p>Non-stationary tides, including river tides and internal tides, have drawn great interest among oceanographers due to their complicated nature and practical importance. To date, diverse theoretical and numerical models have been successfully developed to explore the temporal evolution of non-stationary tides. However, tidal non-stationarity has never been quantified, which impedes the comparative studies of global non-stationary tides under different tidal conditions. In this study, we propose a comprehensive index to quantify tidal non-stationarity in sea areas with different tidal types (river tides and internal tides), tidal ranges (macro, meso, and micro), and tidal regimes (diurnal, semi-diurnal, and mixed) within a unified framework. The proposed non-stationarity index is motivated by the limitations of classical harmonic analysis model in capturing non-stationary tides and integrates several widely-used statistical metrics. Practical applications in the Columbia River Estuary for river tides and in the Timor Passage for internal tides unequivocally demonstrate the reliability of the proposed index. Tidal non-stationarity in the Columbia River Estuary ranges from 0.078 to 0.592, while in the Timor Passage it ranges from 0.583 to 1.149. Evidently, internal tides in the Timor Passage are substantially more non-stationary than tides along the Columbia river channel because of their multi-modal structures and potential influences of multiple physical processes. Nevertheless, cautions should be warranted in the application of the developed index, as it may overestimate tidal non-stationarity in the presence of pronounced high-frequency non-tidal variations.</p>