A box forming device includes an internal partition wall including a positioning area to support a blank arriving from a magazine and positioned between a forming head movable along a rectilinear trajectory to intercept, in an operating position, a portion of the blank. A die receives the blank and forms an open box having a base and side walls defining a housing compartment. The forming head has an operating end for contact with the blank provided with a through seat extending transversely with respect to the rectilinear trajectory. A strap feeding member feeds a strap along a strap along a strap feeding direction in the through seat, which temporarily accommodates the strap. A strap retaining member retains the strap in the through seat while the forming head moves towards the blank, and to release the strap in the compartment such that the strap forms a partition wall which separates two adjacent compartments.