  Optical spectroscopy of ultimately thin materials has significantly enhanced
our understanding of collective excitations in low-dimensional semiconductors.
This is particularly reflected by the rich physics of excitons in atomically
thin crystals which uniquely arises from the interplay of strong Coulomb
correlation, spin-orbit coupling (SOC), and lattice geometry. Here we extend
the field by reporting the observation of room temperature excitons in a
material of non-trivial global topology. We study the fundamental optical
excitation spectrum of a single layer of bismuth atoms epitaxially grown on a
SiC substrate (hereafter bismuthene or Bi/SiC) which has been established as a
large-gap, two-dimensional (2D) quantum spin Hall (QSH) insulator. Strongly
developed optical resonances are observed to emerge around the direct gap at
the K and K' points of the Brillouin zone, indicating the formation of bound
excitons with considerable oscillator strength. These experimental findings are
corroborated, concerning both the character of the excitonic resonances as well
as their energy scale, by ab-initio \emph{GW} and Bethe-Salpeter equation
calculations, confirming strong Coulomb interaction effects in these optical
excitations. Our observations provide the first evidence of excitons in a 2D
QSH insulator at room temperature, with excitonic and topological physics
deriving from the very same electronic structure.
