Excitonic states of tightly bound electron-hole pairs dominate the optical response in a growing class of two-dimensional (2D) materials and their van der Waals (vdW) heterostructures. In transition metal dichalcogenides (TMDs) a useful guidance for the excitonic spectrum is the analogy with the states in the 2D hydrogen atom. From our symmetry analysis and solving the Bethe-Salpeter equations we find a much richer picture for excitons and predict their tunable resonant $s-p$ mixing. The resonance is attained when the sub-band splitting matches the energy difference between the $1s$ and $2p_{+}$ (or $2p_{-}$) excitons, resulting in the anticrossing of the spectral lines in the absorption as a function of the sub-band splitting. By focusing on TMDs modified by magnetic proximity, and gated 3R-stacked bilayer TMD, we corroborate the feasibility of such tunable spin splitting. The resulting tunable and bright $s-p$ excitons provide unexplored opportunities for their manipulation and enable optical detection of Rashba or interlayer coupling in vdW heterostructures.

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• Excitons
• Bilayer films
• Monolayer films
• Transition metal dichalcogenides
• Twisted heterostructures
• Bethe-Salpeter equation