A stackable adapter having opposing ports configured to accept differing fiber optic connectors along a longitudinal axis. The stackable adapter is formed from opposing partially open sidewalls having a recess and a protrusion opposite one another near a top edge and bottom edge. Mating two stackable adapters forms a stackable adapter assembly with stackable outer sidewalls for reducing the width of an array of two stackable adapters by at least one-third over placing two standard adapter side-by-side in the panel.