We report a robust method for calibrating optical tweezers in any viscoelastic medium. This approach uses two coupled measurements-one from a static experiment in which a trapped particle diffuses passively within the tweezer's harmonic potential and another from a dynamic experiment in which the trap is jumped discontinuously to a new position while the particle undergoes transient relaxation back into the minimum of the optical potential. Together, these are sufficient to determine the stiffness of the trap in a material of unknown rheology. The method is tested in a Newtonian fluid and compares favorably with other means of calibration. The calibration is also performed in a non-Newtonian fluid of which standard optical tweezer calibration methods may struggle to characterize. The correctly calibrated optical tweezer microrheometer measures the rheology of polymer solutions in agreement with macrorheological measurements.