The first observation of top quark production in proton-nucleus collisions is reported using proton-lead data collected by the CMS experiment at the CERN LHC at a nucleon-nucleon center-of-mass energy of sNN=8.16  TeV. The measurement is performed using events with exactly one isolated electron or muon candidate and at least four jets. The data sample corresponds to an integrated luminosity of 174  nb-1. The significance of the tt¯ signal against the background-only hypothesis is above 5 standard deviations. The measured cross section is σtt¯=45±8  nb, consistent with predictions from perturbative quantum chromodynamics.