The microwave spectrum of the weakly bound complex Ar-CH3CN has been observed using a pulsed-nozzle Fourier-transform microwave spectrometer. The spectrum is characteristic of an asymmetric rotor with nearly free internal rotation of the methyl group. Spectroscopic constants for the ground internal rotor state, in megaHertz, are A = 9323.7769 (22), B + C = 3439.5578 (15), B-C= 326.6860(12), Δj =0.013 361(14), ΔJK = 0.226 143(99), ΔK = -0.209 86(77), δj = 0.002 532(11), eqQaa = 2.0542(59), eqQbb - eqQcc = - 6.2856(86). The complex has a T-shaped geometry in which the acetonitrile axis is nearly perpendicular to the line joining the centers of mass of the monomers, and the methyl group undergoes nearly free internal rotation. The center-of-mass separation is 3.6505(7) Å and the dipole moment component along the b inertial axis of the complex is 3.802(5) D. The latter is quantitatively accounted for by the sum of the monomer permanent moment and the dipole-induced dipole in the argon.