The C12(p,γ)N13 reaction is the onset process of both the CNO and hot CNO cycles that drive massive star, red and asymptotic giant branch star, and novae nucleosynthesis. The C12(p,γ)N13 rate affects the final abundances of the stable C12,13 nuclides with ramifications for meteoritic carbon isotopic abundances and the s-process neutron source strength. Here, an underground measurement of the C12(p,γ)N13 cross section is reported. The present data, obtained at the Felsenkeller shallow-underground laboratory in Dresden (Germany), encompass the 320-620 keV center of mass energy range to include the wide and poorly constrained E=422 keV resonance that dominates the rate at high temperatures. This work's S-factor results, lower than literature by 25%, are included in a comprehensive R-matrix fit, and the energy of the 12+ first excited state of N13 is found to be 2369.6(4) keV with a radiative and proton width of 0.49(3) eV and 34.9(2) keV, respectively. A reaction rate, based on the present R-matrix fit and extrapolation, is suggested.