One of the possible multi uses of marine space is offshore wind farms and open water aquaculture plans. Unlike coastal installations, offshore wind farms allow for increased availability of wind power and wind persistence, as well as lower visual impact of the turbines. Existing offshore wind farms offer important advantages for aquaculture plans especially in terms of lack of major physical constrains, e.g., navigation routes, submarine cables, marine protected areas. Moreover, enhanced current velocity due to the presence of the piles and to the air fluxes of the turbines may increase the environmental suitability of aquaculture plans in these areas. In addition, the transmission of localized depleted water masses or waste material towards near-shore zones can be avoided, excluding potential impact close to the coast. On the other hand, other environmental constrains (e.g. temperature and salinity variability, dissolved oxygen concentrations, phytoplankton dynamics) need also to be considered when planning aquaculture activities. In this context, the present paper gives a contribution towards the definition of a sustainability index for the large scale localization of marine areas as offshore wind farms and aquaculture plans. This index is developed on the basis of a Multi-Criteria Evaluation (MCE) technique, already used in open water aquaculture localization: raw data were converted to suitability scores, which were combined using additive models, in order to define the overall suitability. A detailed analysis of the environmental suitability is performed for few specific test cases at offshore wind farms located in the Denmark coastal zone, for which some projects of offshore aquaculture plans have been proposed. The use of this sustainability index will perform quantitative evaluations of the degree of compatibility between the offshore wind farms and the open water aquaculture, highlighting the main problems which can rise by the integration of both technologies.