A disk rotating in a viscous fluid decelerates with an angular velocity inversely proportional to time. It is found that the unsteady Navier–Stokes equations admit similarity solutions which depend on a nondimensional parameter S =α/Ω0, measuring unsteadiness. The resulting set of nonlinear ordinary differential equations is then integrated numerically. The special case of S =−1.606 699 corresponds to the decay of rotation of a free, massless disk in a viscous fluid.