A diesel exhaust fluid (DEF) cap assembly for a vehicle includes: a body having a first opening and a second opening arranged at an opposite side of the first opening, the second opening configured to be opened and closed; a flange portion protruding radially outward between the first opening and the second opening of the body, and having a plurality of through holes and a flange gasket configured to be sealably coupled to a mounting subject on which the flange portion is mounted; and a membrane member coupled to the flange portion, and configured to cover the plurality through holes and allow air flow.