Shape optimization of a diffuser with vortex generator on the inlet that mimics the impeller of a Francis turbine was performed. The optimization is based on the use of the adjoint solver in ANSYS Fluent. The main objective was to suppress the vortex rope in off-design operating regimes. An iterative optimization methodology for the adjoint solver was developed. The objective function for the adjoint solver was chosen to minimize the momentum of the flow around the diffuser axis. A new diffuser design was presented and analyzed. Then, a comparison of the vortex rope formation and flow field behavior inside the new and old diffuser designs was performed. It was found that the shape optimization did not lead to any significant improvement in the vortex rope behavior, and the energy of the vortex rope was only transferred further away from the vortex generator hub.