Disclosed are a system and a method for processing boil-off gas. A system for processing boil-off gas, which is generated in an LNG storage tank in a ship or a marine structure, according to the present invention comprises: a regasification line for supplying natural gas by regasifying LNG stored in the LNG storage tank; and a high-pressure compression line for compressing the boil-off gas generated in the LNG storage tank to a high pressure and supplying to a point down-stream from the regasification line.