Abstract: An apartment mailbox signal device to indicate the presence of outgoing mail in an apartment mailbox is comprised of a detachable mounting base having means for engaging a rotatable arm, a generally L-shaped rotatable arm for supporting a signal means, and a signal means connected to the rotatable arm indicating the presence of outgoing mail in the mailbox. The mounting base includes means for selectively securing the mounting base to an interior wall of the apartment mailbox and means for receiving or supporting the rotatable arm. The generally L-shaped rotatable arm is comprised of a vertical pivot member and a horizontal support member. The vertical member is received within the mounting base such that it may pivot about an axis parallel with the interior wall of the mailbox to which the mounting base is secured.