Disclosed is a boil off gas treatment system for a liquefied hydrogen storage tank for a ship. The boil off gas treatment system for a liquefied hydrogen storage tank for a ship includes: a liquefied hydrogen storage tank in which liquefied hydrogen is stored; an LNG storage tank in which LNG is stored; and a boil off gas treatment device treating liquefied hydrogen boil off gas generated in the liquefied hydrogen storage tank and LNG boil off gas generated in the LNG storage tank, wherein the boil off gas treatment device includes: a first heat exchanger supplied with the liquefied hydrogen boil off gas and the LNG boil off gas to heat-exchange the same to each other; and a hydrogen occlusion tank filled with hydrogen occlusion alloy therein and supplied with the liquefied hydrogen boil off gas through the first heat exchanger to store the same.