Fastener (4) for dismountably assembling laminar elements such as the bottom (2) and lateral (3) walls of containers, boxes (1) and the like comprising, at least, a tongue (6) having a widened head (6a) defining a neck (6b), which is located in one of the elements to be assembled and, at least, a groove (5) intended for receiving said neck (6b), located in the other element to be assembled, and whose width is comprised between the width of the neck (6b) and the width of the widened (6a) head; the groove (5) having, at least, an open side (7) where a resiliently displaceable retainer (8) is provided.