$B$-tagging plays a fundamental role at LHC, as it helps in the identification of heavy particles that decay to bottom quarks, as the top quark and the Higgs boson or heavy exotic particles. The Soft Muon Tagger (SMT) allows to identify jets from $b$-quarks taking advantage of the presence of a muon coming from semileptonic decays of $b$-hadrons. The development of this new $b$-tagger is described, showing that, despite the low efficiency of the jet-muon association based on the angular distance, the discriminating power of the associated muon variables is remarkable to reject light jets. An enhanced performance has been reached for all light jet rejection working points by adding the SMT output to the best performing multivariate $b$-tagger in ATLAS (MV2). A good modeling of input and output variables is shown, comparing simulation with Run 2 data.