Abstract: A drain overflow alarm for a washing machine drain, which alarm is mounted in a vertical drain line offset provided with a check valve and extending from the vertical fixed drain line or standpipe of a structure. In a preferred embodiment the alarm includes a float switch located in the top end of the drain line offset and a transformer, relay, alarm and optical reset button, all provided in a control housing. The float switch is electrically connected to the transformer, relay and alarm, such that obstruction of the fixed drain line causes water to rise in the fixed drain line and through the check valve into the drain line offset. The rising water raises the float element of the float switch and closes the switch to activate the transformer relay and alarm, as well as terminate operation of the washing machine.