Computer-assisted method for optimizing surfaces of composite-material structures as part of a design process that includes the following stages: a) Providing a multi-cell surface (11) of the structure obtained using aerodynamic calculations; b) Transforming said multi-cell surface (11) into an optimized surface (13) with fewer cells, concatenating contiguous cells and maintaining point and tangent continuity between them; c) Using said optimized surface (13) as geometric master when designing the components of the structure. The method is particularly applicable to the design of structures with a plurality of components and in particular fuselages of aircraft made of composite material. The invention also relates to a computer program for performing the method.