A universal end cap and associated method are provided for rain gutter debris guards of the type having a top cap extending along a roof and over the open top of an associated gutter. The universal end cap includes a generally flat, rigid cover panel with a substantially rectangular lower portion sized to extend over either end of the guard, and is severable to custom fit the end cap to a wide variety of different gutters. A pair of curved nose portions project outwardly from opposite sides of the cover panel adjacent the upper edge thereof, and are configured to close off the forward portion of the top cap at either end thereof. A mounting flange extends along at least a portion of the upper edge of the cover panel, and a retainer fastens the mounting flange to the top cap at either end thereof to prevent debris form entering between the adjacent ends of the gutter and the guard.