The present study focuses on the evaluation of the impact of marine stable stratification on turbine performance and wake characteristics. Stratification is usually present in regions where marine turbines are installed; this is the case of estuarine basins and shelf seas. Stratification influences the turbine efficiency and rotor wake development; on the other hand, the turbine wake may increase vertical mixing and reduce stratification in the water basin. To evaluate mutual interaction between a marine turbine and vertical stratification, two types of stable stratified conditions are simulated, a weak and a strong stratified condition, respectively. The numerical analysis is carried out using Large Eddy Simulation (LES) coupled with a Blade Element Momentum (BEM) turbine module. The capability of the model in reproducing power and thrust characteristics of a turbine is proved by comparison with experimental data. Results show that stratification has a remarkable impact on wake development: concerning power performance, as the stratification intensifies, it increases due to the growth of streamwise velocity; meanwhile, the power coefficient slightly increases. Also, the recovery of wake velocity deficit is faster in case of strong stratification, thus reducing the extension of the downward region affected by the presence of the turbine. Results also show that the turbine modifies stratification, specifically the mixing effect is higher in case of strong stratification; this phenomenon is ascribed to the strong vertical meandering of the wake and the development of an eddy that overturns high-density fluid over lower-density fluid.