A search is performed for heavy Majorana neutrinos (N) using an event signature defined by two muons of the same charge and two jets ( μ±μ±jj ). The data correspond to an integrated luminosity of 19.7 fb −1 of proton–proton collisions at a center-of-mass energy of 8 TeV, collected with the CMS detector at the CERN LHC. No excess of events is observed beyond the expected standard model background and upper limits are set on |VμN|2 as a function of Majorana neutrino mass mN for masses in the range of 40–500 GeV, where VμN is the mixing element of the heavy neutrino with the standard model muon neutrino. The limits obtained are |VμN|2<0.00470 for mN=90 GeV , |VμN|2<0.0123 for mN=200 GeV , and |VμN|2<0.583 for mN=500 GeV . These results extend considerably the regions excluded by previous direct searches.