The present invention is a substrate for growing a single crystal diamond layer including: at least, a base material made of a single crystal diamond, and an iridium film or a rhodium film heteroepitaxially grown on a side of the base material where the single crystal diamond layer is to be grown; wherein a peripheral end portion of a surface of the base material on the side where the single crystal diamond layer is to be grown is chamfered with a curvature radius (r), the curvature radius satisfying (r)≧50 μm. As a result, there is provided a substrate for growing a single crystal diamond layer and a method for producing a single crystal diamond substrate, the substrate and the method in which a single crystal diamond having uniform and high crystallinity can be reproducibly produced at low cost.