A crossmember assembly for a vehicle may include an upper panel; a pair of lower panels assembled at a lower side of both end portions of the upper panel to form a first box, a center lower panel, a middle portion of which is assembled to a lower middle portion of the upper panel, and a front side lower arm mounting panel and a rear side lower arm mounting panel that are disposed at a lower front side and a lower rear side of the crossmember assembly, respectively, and mounted to the both end portions of the upper panel to house a lower arm.