Abstract The optical activity of an undoped Y3Al5O12 (YAG) crystal is measured in the near infrared at a fixed temperature. The crystal is located inside the bore of a hollow cylindrical magnet of uniform magnetization. By integrating the applied magnetic field along the cylindrical axis of the crystal, the Verdet constant is determined for wavelengths from 1300 nm to 1350 nm as well as at 1064 nm. For the 1300 nm range, the Verdet constant is found to be approximately 1.38 rad / T m while at 1064 nm it is 2.12 rad / T m . From a fit to these measurements, we can extract values for the band gap energy and K-parameter of the crystal. For undoped YAG, we report a band gap value of 8.1 eV ± 0.3 eV and a K-parameter of 596 rad / T m ± 52 rad / T m , consistent with the previous measurements made with visible light.