Maritime transportation still largely relies on fossil fuels, contributing to a plethora of consequences to the environment by increasing greenhouse gas emissions. In recent years a trend towards the use of marine renewable energy sources has been arisen in order to reduce the important environmental impacts. The Greek project NAVGREEN funded by the European Union and Greece, aims inter alia to develop alternative solutions to traditional fuels and improve the energy efficiency in the Greek ships and ports, for shipping to become more sustainable and efficient. Among the resources that are considered are wave and offshore wind energy. In this work, the assessment of the available wave and offshore wind power potential along two coastal maritime routes of the Greek Seas is performed. Offshore wind can serve as an alternative power source either for ship propulsion (wind assisted propulsion systems) or for lighting, operation of electronic devices and navigation. For the wind and wave climate analysis along the routes, long – term offshore data are obtained from the reanalysis wind dataset CERRA and the wave dataset MEDSEA_HINDCAST_WAV_006_012, respectively. The shipping routes that are examined are Chios – Oinousses in eastern Aegean Sea, and Piraeus – Aegina Isl., in Saronic Gulf. From the numerical results, it is evident that the offshore power potential along Chios – Oinousses, is significantly greater compared to Piraeus-Aegina. In general, the Greek Seas are characterized by a great offshore wind and fair amount of wave power potential, that may be used as additional energy sources to maritime transportation.