Sea defences are at increasing risk of wave overtopping due to changing hydraulic conditions caused by climate change and deteriorating structural resistance. This paper presents a method for assessing wave overtopping risk of coastal flood defences subjected to changing operational conditions. The joint probability of hydraulic parameters such as sea water level and significant wave height is investigated using the extreme value analysis approaches taking into account rising sea level in the future. Meanwhile, structural resistance degradation such as the crest level settlement of earth sea dykes is considered in overtopping risk analysis by utilising a state-based stochastic deterioration model. Time-dependent probability of wave overtopping failure is estimated from the associated limit state equation, and the effect of hydraulic parameters and crest level settlement on the risk of overtopping failure is investigated. A risk cost optimised maintenance strategy is then determined by balancing the risk of overtopping failure and the costs for structural maintenances. The results for the numerical example of a typical earth sea dyke show that the probability of overtopping failure will increase significantly due to changing hydraulic parameters and losing crest level, thus appropriate maintenance will be needed to reduce the risk during the lifecycle.