We study the dynamics of coupled phase oscillators on a two-dimensional Kuramoto lattice with periodic boundary conditions. For coupling strengths just below the transition to global phase-locking, we find localized spatiotemporal patterns that we call "frequency spirals." These patterns cannot be seen under time averaging; they become visible only when we examine the spatial variation of the oscillators' instantaneous frequencies, where they manifest themselves as two-armed rotating spirals. In the more familiar phase representation, they appear as wobbly periodic patterns surrounding a phase vortex. Unlike the stationary phase vortices seen in magnetic spin systems, or the rotating spiral waves seen in reaction-diffusion systems, frequency spirals librate: the phases of the oscillators surrounding the central vortex move forward and then backward, executing a periodic motion with zero winding number. We construct the simplest frequency spiral and characterize its properties using analytical and numerical methods. Simulations show that frequency spirals in large lattices behave much like this simple prototype.Spirals have long been objects of fascination in many parts of human culture, from art and architecture to science and mathematics. Within nonlinear dynamics, spirals have been studied in connection with such diverse phenomena as spiral density waves in galaxies, spiral waves of electrical activity in the heart and nervous system, growth spirals caused by screw dislocations in crystals, and spiral patterns of florets on the heads of sunflowers, to name just a few. Here we consider a spiral pattern of a type that, as far as we know, has not been discussed previously. Instead of a pattern of density or phase, it is a pattern of instantaneous frequencies: hence, a "frequency spiral." We found it in a two-dimensional array of coupled oscillators whose natural frequencies were not quite identical, a regime in which the oscillators could no longer maintain global frequency entrainment and instead self-organized into one or more frequency spirals. We explore the properties of frequency spirals by analytical and geometrical methods and illustrate them by videotaped simulations of their dynamics in time and space.