Lepton flavor universality ratios have been the source of much interest in the search for beyond the Standard Model physics, with longstanding anomalies in the R(D(*)) measurements at 3.8 standard deviations above the Standard Model value. A measurement of the ratio of semileptonic branching fractions R(J/psi), defined as R}(J/psi) = B(B_c^+ \to J/psi tau^+ nu_{tau})/B(B_c^+ \to J/\psi mu^+ nu_{mu}), is reported using a sample of proton-proton collision data corresponding to an integrated luminosity of 5.4fb^{-1} (Run-2) recorded by the LHCb experiment in 2016--2018 at a center-of-mass energy of 13 TeV.