Using Penning trap mass spectrometry, we have performed a precise determination of the Q value for the highly forbidden β decay of Cd113. An independent measurement of the Q value fixes the end-point energy in a fit to the Cd113β-decay spectrum. This provides a strong test of systematics for detectors that have observed this decay, such as those developed for ββ-decay searches in cadmium and other isotopes. It will also aid in the theoretical description of the β-decay spectrum. The result, Qβ=323.89(27)keV, agrees at the 1.3σ level with the value obtained from the 2012 Atomic Mass Evaluation [Chin. Phys. C 36, 1603 (2012)1674-113710.1088/1674-1137/36/12/003], but is a factor of almost four more precise. We also report improved values for the atomic masses of Cd113,In113, and Cd112.