The OPERA experiment is designed to search for νμ→ντ oscillations in appearance mode, i.e., through the direct observation of the τ lepton in ντ-charged current interactions. The experiment has taken data for five years, since 2008, with the CERN Neutrino to Gran Sasso beam. Previously, two ντ candidates with a τ decaying into hadrons were observed in a subsample of data of the 2008–2011 runs. Here we report the observation of a third ντ candidate in the τ−→μ− decay channel coming from the analysis of a subsample of the 2012 run. Taking into account the estimated background, the absence of νμ→ντ oscillations is excluded at the 3.4 σ level.