Abstract Taking advantage of large top production rates at the LHC, the leptonic final states with J/ ψ are explored for a precise determination of the top quark mass. The top is partially reconstructed by combining the isolated lepton from the W with the J/ ψ from the decays of the corresponding b-quark. The method relies, to a large extent, on the proper Monte Carlo description of top production and decay. The main emphasis is put on the expected systematics uncertainties. Mass measurement accuracy is dominated by the current understanding of theoretical uncertainties which result in a systematic error of ≲ 1 GeV.