Ammonium iodate, NH4IO3, is pyroelectric at room temperature and crystallizes in the orthorhombic system with space groupPc21nand four formulas in the unit cell. The lattice constants at 297°K area = 6.4115 ± 0.0005, b = 9.1706 ± 0.0005, andc = 6.3740 ± 0.0005 Å. The integrated intensities of 4591 reflections within a hemisphere of reciprocal space of radius(sin&thgr;) / &lgr; = 1.02 Å−1were measured using PEXRAD. The crystal structure was solved by Patterson and Fourier series and refined by the method of least‐squares using 1458 symmetry independentFmeas. The final agreement factorRis 0.054, based on a model with anisotropic iodine and oxygen and isotropic nitrogen thermal vibrations, and with a spherical distribution of hydrogen around the nitrogen atom. The structure consists of discrete IO3−pyramidal ions arranged to give a highly distorted octahedral environment about the iodine. The IO3−ions and NH4−ions form chains parallel to the polar axis. Three I&sngbnd;O distances are in the range 1.765–1.836 Å and three more are in the range 2.778–2.830 Å. There are four N···O distances under 3 Å, indicative of weak hydrogen bonding. The structure is a highly distorted perovskite, and is potentially ferroelastic. The spontaneous strain at 297°K is 0.00293. The spontaneous polarization, calculated on the basis of point charges on the two ions, is 1.9 × 10−2C m−2.