The observation of neutrino oscillations and masses motivates extensions of the Standard Model containing right-handed neutrinos. If additional new physics exists at scales beyond the direct reach of present experiments, its effects can be systematically described within the neutrino Standard Model Effective Field Theory (νSMEFT). In this framework, heavy neutral leptons provide a promising target for collider searches. In this work, we reinterpret a CMS search for heavy Majorana neutrinos in the same-sign dimuon plus two jets final state to constrain the parameter space of the dimension-six νSMEFT. Using a detailed recast of the experimental analysis, we present results for both a pure-agnostic benchmark and a benchmark incorporating neutrinoless double-beta-decay bounds. For heavy-neutrino masses in the range 200 GeV≤mN≤15 TeV, the observed upper limits on the effective coupling strength range from 5.8×10−7GeV−2 to 2.3×10−6GeV−2. These results constitute the first experimental exclusion limits on the agnostic νSMEFT parameter space obtained from a dedicated recast of an LHC heavy-neutrino search.