Marine renewable energy (MRE) is arguably the most underdeveloped form of renewable energy. However, its estimated potential as a renewable energy source is only next to solar energy, and hence, it can replace the existing fossil-based energy sources. Five different forms of MRE, viz., tidal barrage, tidal and ocean currents, wave power, ocean thermal energy conversion (OTEC), and salinity gradient are discussed in detail in this chapter. The technology, existing plants and their operation, estimated resource potential, future developments, advantages and challenges of MRE are presented. Offshore areas can also be used to harness renewable energy from the wind and sun and recent developments in deployment of offshore wind farms and solar farms are analysed. Marine biomass is a potential source of biofuels, and some aspects of their use as future sources of energy are also discussed. Global efforts to harness renewable energy from oceans are ongoing and if successfully deployed at a large scale and low cost, these technologies can complement renewable energy generation on land thereby contributing to sustainable energy security in the long term.