A floor panel assembly including a first and a second panel. A first edge portion of the first panel includes an upwardly extending locking element and a second edge portion of the second panel includes a downwardly open locking groove configured to cooperate with the locking element for horizontally locking the first and second edge portions to each other. The floor panel assembly further includes a connecting device configured to be at least partially arranged in the locking groove in a locked state of the first and second edge portions. The connecting device includes a positioning member configured to extend along a third edge portion of the first panel in the locked state, wherein the first and third edge portions join each other at a corner portion of the first panel. Also, a corresponding connecting device.