Provided are: a GaN substrate wafer having a crystallinity suitable as a substrate for a semiconductor device as well as an improved productivity; and a method of producing the same. The GaN substrate wafer is a (0001)-oriented GaN substrate wafer that includes a first region arranged on an N-polar side and a second region arranged on a Ga-polar side via a regrowth interface therebetween. In this GaN substrate wafer, the second region has a minimum thickness of not less than 20 μm, the concentration of at least one element selected from Li, Na, K, F, Cl, Br, and I in the first region is 1×1015 atoms/cm3 or higher, and the second region satisfies one or more conditions selected from the following (a) to (c): (a) the Si concentration is 5×1016 atoms/cm3 or higher; (b) the O concentration is 3×1016 atoms/cm3 or lower; and (c) the H concentration is 1×1017 atoms/cm3 or lower.