Abstract: An adjustable adapter, which permits any portable box fan to be operated while sitting on any household windowsill. Many home or apartment windowsills are too narrow (front to back) for the box fan to stand alone. The adapter provides the needed support, even on sills as narrow as 2" or with windows that do not have a vertical sliding sash for support. An adapter assembly (32) consists of a flat rigid base member (12) which is directly connected to the enclosure of a box fan (14) with hook and loop fasteners (10 and 10A). A protruding arm (16) is connected at one end to base member (12). A contact finger (24) on the opposite end of arm (16) is placed into a window frame track (28) to support box fan (14). A secondary protruding arm (16A) is used to retain a window curtain (38) out of the air intake. This is done by placing curtain (38) between arm (16) and arm (16A). Secondary arm (16 A) is also used to instantly support fan (14) when fan (14) is rotated 180.degree. on windowsill (30).