  We discuss weak kaon decays in a scenario in which the Standard Model is
extended by massive sterile fermions. After revisiting the analytical
expressions for leptonic and semileptonic decays we derive the expressions for
decay rates with two neutrinos in the final state. By using a simple effective
model with only one sterile neutrino, compatible with all current experimental
bounds and general theoretical constraints, we conduct a thorough numerical
analysis which reveals that the impact of the presence of massive sterile
neutrinos on kaon weak decays is very small, less than $1\%$ on decay rates.
The only exception is $\mathcal{B} (K_L\to \nu\nu)$, which can go up to
$\mathcal{O}( 10^{-10})$, thus possibly within the reach of the KOTO
experiment. In other words, if all the future measurements of weak kaon decays
turn out to be compatible with the Standard Model predictions, this would not
rule out the existence of massive light sterile neutrinos with non-negligible
active-sterile mixing. Instead, for a sterile neutrino of mass below $m_K$, one
might obtain a huge enhancement of $\mathcal{B} (K_L\to \nu\nu)$, otherwise
negligibly small in the Standard Model.
