  We study the sensitivity of future electron-positron colliders to UV
completions of the fermionic Higgs portal operator $H^\dagger H \bar \chi
\chi$. Measurements of precision electroweak $S$ and $T$ parameters and the
$e^+e^- \to Zh$ cross section at the CEPC, FCC-ee, and ILC are considered. The
scalar completion of the fermionic Higgs portal is closely related to the
scalar Higgs portal, and we summarize existing results. We devote the bulk of
our analysis to a singlet-doublet fermion completion. Assuming the doublet is
sufficiently heavy, we construct the effective field theory (EFT) at
dimension-6 in order to compute contributions to the observables. We also
provide full one-loop results for $S$ and $T$ in the general mass parameter
space. In both completions, future precision measurements can probe the new
states at the (multi-)TeV scale, beyond the direct reach of the LHC.
