The selection of suitable sites for offshore wind power plants is an important marine spatial planning problem in the Baltic Sea. The application of multiple and different decision support tools (DSTs) can facilitate site selection. This study employs a geographical information system (GIS) integrated with the levelized cost of energy (LCOE) model, and GIS with multi-criteria decision analysis (MCDA) using an analytical hierarch process (AHP), fuzzy logic, weighted linear combination (WLC), and the technique for order of preference by similarity to ideal solution (TOPSIS) as an extension of MCDA. The core novelty of the study is the use of several concurrent state-of-the art techniques to quantify the detailed spatial pattern of suitability of different sea areas for offshore wind power plant installations for the entire Baltic Sea at an unprecedented, for such a large sea area, resolution. Important parameters were identified and data from the Global Wind Atlas, HELCOM, and EMODnet portals were used. The most appropriate areas exhibit LCOE values between 42 and 58 €/MWh, MCDA values in the range of 0.78–0.92, and capacity factor values from 0.52 to 0.55. The areas identified as the least appropriate were characterized by LCOE values ranging from 107 to 210 €/MWh, MCDA values between 0.34 and 0.59, and capacity factor values within the range of 0.30–0.43. The outcomes using both methodologies were similar. The decision preferred are mostly locations close to shore in shallow water and with a high capacity factor. The largest suitable areas are in the south-west Baltic Sea whereas some suitable locations are identified further to the north and east.