An object is to provide a thread forming tap configured to obtain an inside diameter dimension of a female thread having a targeted processing accuracy more easily than in related art. A thread forming tap (1) is a cold forming tap for forming a female thread by causing plastic deformation of a prepared hole of a workpiece, and includes a male thread portion (3) that protrudes in a thread shape in a radial direction. The male thread portion includes at least one protruding portion (5), which includes a plurality of first thread ridges (51) arranged in an axial direction and that protrude in the radial direction and a first root (52) formed between the first thread ridges adjacent to each other in the axial direction, and at least one adjustment portion (6), which is arranged with the at least one protruding portion in one lead of the male thread portion. A distance from an axial center to a section of the adjustment portion corresponding to the first root in the one lead is longer than a root diameter of the first root and shorter than a ridge diameter of the first thread ridge adjacent to the first root.