A search is presented for the production of a Higgs boson in association with a single top quark, based on data collected in 2016 by the CMS experiment at the LHC at a center-of-mass energy of 13 TeV, which corresponds to an integrated luminosity of 35.9 fb . The production cross section for this process is highly sensitive to the absolute values of the top quark Yukawa coupling, , the Higgs boson coupling to vector bosons, , and, uniquely, to their relative sign. Analyses using multilepton signatures, targeting , , and decay modes, and signatures with a single lepton and a pair, targeting the decay, are combined with a reinterpretation of a measurement in the channel to constrain . For a standard model-like value of , the data favor positive values of and exclude values of below about -0.9\,{y_\mathrm{t}} ^\mathrm{SM}.