  Two-dimensional (2D) transition-metal dichalcogenides (TMDC) are considered
highly promising platforms for next-generation optoelectronic devices. Owing to
its atomically thin structure, device performance is strongly impacted by a
minute amount of defects. Although defects are usually considered to be
disturbing, defect engineering has become an important strategy to control and
design new properties of 2D materials. Here, we produce single S vacancies in a
monolayer of MoS$_2$ on Au(111). Using a combination of scanning tunneling and
atomic force microscopy, we show that these defects are negatively charged and
give rise to a Kondo resonance, revealing the presence of an unpaired electron
spin exchange coupled to the metal substrate. The strength of the exchange
coupling depends on the density of states at the Fermi level, which is
modulated by the moir\'e structure of the MoS$_2$ lattice and the Au(111)
substrate. In the absence of direct hybridization of MoS$_2$ with the metal
substrate, the S vacancy remains charge-neutral. Our results suggest that
defect engineering may be used to induce and tune magnetic properties of
otherwise non-magnetic materials.
