Well-based diffusers have greatly developed since the introduction of Schroeder diffusers forty years ago. Nevertheless, common designs found nowadays still have a lack of efficiency, especially at low frequencies when arranged in arrays. The present study therefore aimed at proposing a method to design optimal well-based sound diffusers. Based on simulations performed with the boundary-element method, the different effects of the parameters determining the geometry of a well-based diffuser were first investigated so as to identify the key variables to take into consideration. According to these findings, an optimization strategy was adopted and optimal designs were suggested, which were then compared with a chosen reference diffuser. Finally, a method to improve the poor performance of one of the optimal designs in a specific frequency band was introduced.