  We investigate excitonic transitions in a h-BN encapsulated monolayer
$\textrm{MoS}_2$ phototransistor by photocurrent spectroscopy at cryogenic
temperature (T = 5 K). The spectra presents excitonic peaks with linewidths as
low as 8 meV, one order of magnitude lower than in earlier photocurrent
spectroscopy measurements. We observe four spectral features corresponding to
the ground states of neutral excitons ($\textrm{X}_{\textrm{1s}}^\textrm{A}$
and $\textrm{X}_{\textrm{1s}}^\textrm{B}$) and charged trions
($\textrm{T}^\textrm{A}$ and $\textrm{T}^\textrm{B}$) as well as up to eight
additional spectral lines at energies above the
$\textrm{X}_{\textrm{1s}}^\textrm{B}$ transition, which we attribute to the
Rydberg series of excited states of $\textrm{X}^\textrm{A}$ and
$\textrm{X}^\textrm{B}$. The relative intensities of the different spectral
features can be tuned by the applied gate and drain-source voltages, with
trions and Rydberg excited states becoming more prominent at large gate
voltages. Using an effective-mass theory for excitons in two-dimensional
transition-metal dichalcogenides we are able to accurately fit the measured
spectral lines and unambiguously associate them with their corresponding
Rydberg states. The fit also allows us to determine the quasiparticle bandgap
and spin-orbit splitting of monolayer $\textrm{MoS}_2$, as well as the exciton
binding energies of $\textrm{X}^\textrm{A}$ and $\textrm{X}^\textrm{B}$.
