Transition-metal chalcogenide monolayers, with their intrinsic out-of-plane asymmetry and strong spin-orbit coupling, provide an exceptional platform for emergent excitonic and valley-dependent optoelectronic phenomena. Here, we present a comprehensive many-body investigation of the electronic structure and excitonic properties of the CrSO Janus monolayer using density functional theory, quasiparticle $G_0{W}_0$ calculations, and the spinorial Bethe-Salpeter equation. The quasiparticle band structure reveals an indirect fundamental gap of 0.94 eV and a direct optical gap of 1.97 eV, highlighting pronounced many-body renormalization effects. Spinorial $G_0{W}_0\_\mathrm{BSE}$ calculations uncover a strongly correlated excitonic manifold composed of both dark and bright excitons with large binding energies. In particular, a spin-forbidden dark exciton ${\xi }_0^d$ emerges at $1.58\mathrm{eV}$, followed by a spin-allowed bright exciton ${\xi }_1^b$ at $1.70\mathrm{eV}$, yielding a sizable spin-orbit-induced dark-bright splitting of 119 meV. Analysis of the excitonic spin texture reveals a dominant triplet character for the dark state and a mixed singlet-triplet configuration for the bright exciton. The reciprocal-space exciton wave functions exhibit strong valley localization around the $K^{+}$ and $K^{-}$ valleys, demonstrating the valley-selective nature of the optical transitions. Furthermore, electron-hole interactions dramatically reshape the optical response, enhancing the absorption intensity by up to 7.4 times relative to the independent-particle picture and generating broadband optical activity spanning 1.77–3.10 eV across the visible and near-ultraviolet regions. These results establish the CrSO Janus monolayer as a promising excitonic material for next-generation valleytronic, optoelectronic, and visible-light-driven energy-conversion technologies.

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• Chalcogenides
• Janus materials
• Monolayer films
• Density functional calculations
• GW method