We report time-resolved optical pump-terahertz third-harmonic generation spectroscopy of nonthermal manybody charge dynamics in high-purity silicon at room temperature. Although in thermal equilibrium excitons in silicon are unstable due to a lower binding energy than the thermal energy at room temperature, following pulsed non-resonant optical pumping we can identify the formation of transient excitonic states on the picosecond time scale in our high-resolution nonlinear spectroscopic measurements. In contrast to unbound electron-hole pairs, the nonthermal many-body excitonic dynamics feature a characteristic time-dependent evolution, a distinct dependence on charge density, and a nonperturbative nonlinear terahertz response.