The ratio of branching fractions of the decays $\Lambda b0\to pK-e+e-$ and $\Lambda b0\to pK-\mu +\mu -$, $RpK-1$, is measured for the first time using proton-proton collision data corresponding to an integrated luminosity of 4.7 $fb-1$ recorded with the LHCb experiment at center-of-mass energies of 7, 8 and 13 TeV. In the dilepton mass-squared range $GeV2/c4$ and the $pK-$ mass range $MeV/c2$, the ratio of branching fractions is measured to be $RpK-1=1.17-0.16+0.18\pm 0.07$, where the first uncertainty is statistical and the second systematic. This is the first test of lepton universality with b baryons and the first observation of the decay $\Lambda b0\to pK-e+e-$.