<p>Investigating the spatiotemporal evolution and convergence dynamics of ecological efficiency is essential for assessing sustainable development and improving coastal ecological governance. Using panel data from 53 Chinese coastal cities from 2000 to 2022, this study applies a Super-efficiency SBM model with undesirable outputs, kernel density estimation (KDE), standard deviation ellipse (SDE) analysis, spatial autocorrelation, Markov chain analysis, and a panel Tobit model to examine spatial patterns, temporal evolution, and influencing factors of ecological efficiency. The results show that: (1) from 2000 to 2022, ecological efficiency in China’s coastal cities followed an overall evolutionary trajectory characterised by a “fluctuating increase–fluctuating decline–oscillatory recovery” pattern. Spatially, a pronounced pattern of “higher in the north and lower in the south, stronger in the east and weaker in the west” is observed, with agglomeration evolving from point-based clusters to belt-like and areal formations; moreover, the three major marine economic circles exhibit markedly distinct evolutionary trajectories of ecological efficiency. (2) Ecological efficiency in coastal cities demonstrates significant spatial dependence and club convergence. High-efficiency areas exhibit pronounced agglomeration tendencies, whereas low-efficiency areas display certain spillover effects; however, the overall level of regional coordinated development remains inadequate. (3) The determinants of ecological efficiency exhibit pronounced regional heterogeneity. Economic development and openness to the outside world significantly promote improvements in ecological efficiency, whereas policy support exerts an inhibitory effect in most regions. Although industrial structure and technological innovation do not pass conventional significance tests, their potential roles in long-term green transformation should not be overlooked.</p>