An opening / closing structure for a vehicular tray includes a housing opened at a front surface, and internally defining a receiving space, the housing being mounted at a predetermined position of a vehicle body, and having a pair of through-holes formed at both lateral sides of the open front surface, a door covering the open front surface of the housing, a pair of sliding units mounted at both lateral outer surfaces of the housing, the sliding units being horizontally movable through gear engagement, and a pair of connecting members, a first end of each of the connecting members being coupled to the door, a second end of the connecting member passing through one of the through-holes and being fixed to an associated one of the sliding unit.