  A detailed understanding of charged defects in two-dimensional semiconductors
is needed for the development of ultrathin electronic devices. Here, we study
negatively charged acceptor impurities in monolayer WS$_2$ using a combination
of scanning tunnelling spectroscopy and large-scale atomistic electronic
structure calculations. We observe several localized defect states of
hydrogenic wave function character in the vicinity of the valence band edge.
Some of these defect states are bound, while others are resonant. The resonant
states result from the multi-valley valence band structure of WS$_2$, whereby
localized states originating from the secondary valence band maximum at
$\Gamma$ hybridize with continuum states from the primary valence band maximum
at K/K$^{\prime}$. Resonant states have important consequences for electron
transport as they can trap mobile carriers for several tens of picoseconds.
