An extendable window prop for supporting double-hung windows includes a pole that is of adjustable length. A crossbar is coupled to and extends perpendicularly from an upper end of the pole. A first rod is coupled proximate to the upper end of the pole and extends substantially perpendicularly from the pole. The first rod is substantially perpendicular to the crossbar. A second rod is coupled to and extends substantially perpendicularly from the first rod distal from the pole. The second rod is transverse to the pole. The crossbar is positioned on the pole such that the crossbar is configured to position a first window. The first window is supported by the pole. The second rod is positioned on the first rod such that the second rod is positioned to abut a frame of the first window that is positioned on the crossbar.