The fast-spin dynamics of transition-metal ions in an aqueous solution at room temperature has been directly observed in the subnanosecond region by polarization spectroscopy using the pump-probe technique. The time evolution of the optically induced magnetization is monitored through the change in the polarization of the probe pulse. Quantum-beat free-induction decay signals in the ground state of copper ions in an aqueous solution of copper sulfate are observed in transverse magnetic fields. The Fourier transform of the observed signals gives the ESR spectra. From the magnetic-field dependence of the beat frequency, the g-value of copper ions is g=2.20±0.05. The observed line shape of the ESR spectra suggests that the hydrated copper complex rotates in the picosecond region. This all-optical method can be a powerful tool for the study of fast-spin dynamics in solutions.