The pure rotational spectrum of the iron-carbonyl radical FeCO produced by a de discharge of iron pentacarbonyl Fe(CO)(5) was observed in the millimeter-wave (MMW) region. Ten rotational transitions, split into triplets by the electron spin-spin interaction, were assigned to the electronic ground state of FeCO, definitely confirming that the FeCO radical has (3) Sigma- symmetry. Molecular constants in the ground state were derived from an analysis of the MMW spectrum combined with the rotational lines in the lowest spin component Omega = 0 observed by Fourier transform microwave (FTMW) spectroscopy. The rotational and centrifugal distortion constants were determined as B-0 = 4363.883 42(40) MHz and D-0 = 1.217 99(84) kHz. The spin-spin coupling constant and its centrifugal distortion correction are lambda(0) = 684.470(51)GHz and lambda(D) = -258.08(17) kHz, respectively, and the spin-rotation coupling constant is gamma(0) =- 1127.80(35) MHz, where the figures in parentheses are standard errors to lie attached to the last digit.