We theoretically propose a new method for generating up-converted coherent light from two-level systems (TLSs) coupled with a plasmonic nanocavity. The emission spectrum of a TLS excited by a strong laser exhibits a triplet structure called the Mollow triplet. If the lower Mollow sideband is tuned to the cavity mode energy, population inversion of a TLS occurs. When the driving laser is abruptly truncated under this condition, an up-converted photon is emitted from the TLSs. We also predict the up-converted superfluorescence from an ensemble of TLSs as a correlation effect among the excited states of the TLSs.