pyMSER is a Python library that implements the Marginal Standard Error Rule (MSER) to automatically detect the equilibration period in molecular simulations. It identifies the optimal truncation point where a system reaches a steady state, ensuring objective and reproducible calculation of thermodynamic properties. By effectively separating transient data from production data, pyMSER helps reduce computational costs and improves the accuracy of ensemble averages. The library is designed to be applicable to any time series analysis requiring steady-state detection, independent of the simulation method.