Margin lateral erosion is arguably the main mechanism leading to marsh loss in estuaries and lagoons worldwide. Our understanding of the mechanisms controlling marsh edge erosion is currently quite limited and current predictive models rely on empirical laws with limited general applicability. We propose here a simple theoretical treatment of the problem based on dimensional analysis. The identification of the variables controlling the problem and the application of Buckingham's theorem show, purely on dimensional grounds, that the rate of edge erosion and the incident wave power density are linearly related. The predictive ability of the derived relationship is then evaluated, positively, using new long‐term observations from the Venice lagoon (Italy) and by re‐interpreting data available in previous literature. Key Points Marsh edge erosion rate is a linear function of wave power density Marsh edge erosion rate is a function of marsh cliff height Traditional, power‐law, formulas are inconsistent with theory and observations