  Standard thermal leptogenesis in the type-I seesaw model requires very heavy
right-handed neutrinos (RHNs). This makes it hard to probe this scenario
experimentally and results in large radiative corrections to the Higgs boson
mass. In this paper, we demonstrate that the situation is considerably
different in models that extend the Higgs sector by a real scalar singlet.
Based on effective-theory arguments, the extra scalar is always allowed to
couple to the heavy neutrinos via singlet Yukawa terms. This opens up new RHN
decay channels leading to larger $CP$ violation as well as to a stronger
departure from thermal equilibrium during leptogenesis. As a consequence, the
baryon asymmetry can be generated for a lightest RHN mass as low as 500 GeV and
without the need for a highly degenerate RHN mass spectrum. In fact, the
requirement of successful leptogenesis via the Higgs portal coupling singles
out an interesting parameter region that can be probed in on-going and future
experiments. We derive a semianalytical fit function for the final baryon
asymmetry that allows for an efficient study of parameter space, thus enabling
us to identify viable parameter regions. Our results are applicable to a wide
range of models featuring an additional real scalar singlet.
