Abstract: A corner radius tool is provided for use in applying plaster material or the like to an inside corner joint between drywall panels to form a controlled and smoothly radiused surface. The tool includes a tool head in the form of a forwardly open resilient cup carried at a forward end of a tool handle, wherein the cup is adapted to support a quantity of the plaster material as the handle is manipulated to press and spread the plaster material along the inside corner joint. The resilient cup deforms upon engagement with the drywall panels at the corner joint to spread the plaster material with a desired and smoothly radiused surface. The specific radiused geometry of the plaster material surface is variably controlled by varying the orientation of and manual pressure applied to the resilient cup.