Diffusers are the integral parts of many engineering applications and flow systems. An improperly designed diffuser may cause flow separation and low efficiency. The following study presents an approach of a single-point aerodynamic shape optimization of diffusers in turbulent flow. The objective in this study is to maximize the pressure recovery by shaping the diffuser's geometry, taking into account geometrical and aerodynamic constraints. Mesh warping and geometry parametrization is accomplished by fitting the multi-block structured grid to a B-spline volumes and performing the mesh movements by using surface control points embedded with free-form deformation (FFD) volumes. The aerodynamic model solves the RANS equations with SpallartAlmaras turbulence model. A gradient based optimization algorithm is used with an adjoint method in order to compute the objectives and constraints derivatives with respect to the design variables. The single-point optimization simulation increased the pressure recovery coefficient by 12% using 46 design variables. The effect of varying the number of shape design variables is examined. Also, nearly identical configuration is obtained while starting the simulation with a deformed geometry, and the final optimized pressure recovery values differ by 0.8% only. The accuracy and scalability of the presented method make it possible perform shape design optimization for diffusers characterized with complicated geometries under various flow conditions.