Floating solar photovoltaics in water bodies is a novel clean energy technology which has been developed rapidly during the last decade. The current work investigates the possibility and the potential of installing floating photovoltaic systems in the existing hydropower plants in Greece. Studies related with the use of floating photovoltaics in water reservoirs in Greece are limited so far. The characteristics of the existing 24 hydropower plants in Greece have been used for the estimation of the solar photovoltaic systems which can be installed in their water reservoirs. It has been found that the nominal power of these solar energy systems which can be installed in their water reservoirs, covering 10% of their water surface, is at 3,861 MWp while the annual generated electricity at 5,212.35 GWh corresponding at 10.04 % of the annual electricity demand in the country. The capacity factor of the integrated solar and hydro power systems is increased by more than 20%. The research indicates that the existing hydropower plants in Greece can host, in their water dams, floating photovoltaic systems generating significant amounts of green electricity while they also result in many environmental benefits. These novel solar energy systems can contribute, together with other benign energy technologies, in the achievement of the national and EU target for net zero carbon emissions by 2050.