  FeSe$_{1-x}$S$_{x}$ superconductors are commonly considered layered van der
Waals materials with negligible inter-layer coupling. Here, using inelastic
neutron scattering to study spin excitations in single-crystal samples, we
reveal that the magnetic coupling between adjacent Fe layers is not only
significant, as it affects excitations up to \textcolor{black}{15} meV, but
also ferromagnetic in nature, making the system different from most
unconventional superconductors including iron pnictides. Our observation
provides a new standpoint to understand the absence of magnetic order in
FeSe$_{1-x}$S$_{x}$. Since intercalating between the Fe layers is known to
enhance superconductivity and suppress the inter-layer coupling,
superconductivity appears to be a more robust phenomenon in the two-dimensional
limit than antiferromagnetic order.
