A bstract A measurement of the $ \Lambda_{\mathrm{b}}^0 $ lifetime using the decay $ {{{\Lambda_{\mathrm{b}}^0\to J}} \left/ {{\psi \varLambda }} \right.} $ in protonproton collisions at $ \sqrt{s}=7 $ TeV is presented. The data set, corresponding to an integrated luminosity of about 5 fb −1 , was recorded with the CMS experiment at the Large Hadron Collider using triggers that selected dimuon events in the J/ ψ mass region. The $ \Lambda_{\mathrm{b}}^0 $ lifetime is measured to be 1.503 ± 0.052 (stat.) ± 0.031 (syst.) ps.