Abstract: Mailbox signal device is comprised essentially of two plates, a support plate that mounts onto a postbox by integral plate tab elements that position beneath the postbox latch member and a signal plate that is (a) at one end pivotally fixed to the support plate, (b) biased to an upright position, (c) has an extending tab member at its other end that operatively associates with the postbox closure to be retained in a folded position and (d) automatically is biased to said upright position when the tab member is freed by opening of said closure.