The present invention is a base material for forming a single crystal diamond comprising, at least, a seed base material of a single crystal and a thin film heteroepitaxially grown on the seed base material, wherein the seed base material is a single crystal diamond and the thin film is Iridium film or Rhodium film. As a result, there is provided a base material for forming a single crystal diamond that enables a single crystal diamond having a high crystallinity to be heteroepitaxially grown thereon and that can be reused repeatedly and a method for producing a single crystal diamond that enables a single crystal diamond having a high crystallinity and a large area to be produced at low cost.