An electron-spin-resonance absorption spectrum taken at 3-cm wavelength and attributed to trivalent platinum (5d7,t6e) at octahedral sites in yttrium aluminum garnet has been observed at temperatures below 120°K. The spectrum is described by S=12 and I=12 (due to the 34% abundant 195Pt). There are 24 magnetically inequivalent orientations of the platinum ion per unit cell arising from what is believed to be static Jahn-Teller distortions at each of the eight inequivalent octahedral sites. The g and A tensors are orthorhombic with principal components gi=1.962, gj=2.361, gk=2.424 and Al=99 Oe, Am=88 Oe, and An=30 Oe. The two tensors are not diagonal in the same coordinate system and their orientations with respect to the garnet cubic directions is discussed.