Hexagonal boron nitride (hBN) is a semiconductor with unique electronic properties, including a wide bandgap, low dielectric constant, high thermal conductivity, and chemical inertness. Despite its simple crystal structure, the nature of its bandgap remains controversial, with ab initio calculations predicting an indirect bandgap and optical measurements suggesting a direct one. This paper demonstrates that hBN has an indirect bandgap at 5.955 eV, and that its optical properties are significantly influenced by phonon-assisted transitions. Using two-photon spectroscopy, the authors reveal previously unobserved lines, including a weak doublet around 5.93 eV and a dim emission at 5.955 eV, corresponding to the indirect exciton. The study also identifies the corresponding phonon modes and measures the energy splitting between the 1s and 2p exciton states for the first time in an indirect bandgap semiconductor, estimating the exciton binding energy to be 128±15 meV, indicating that excitons in hBN are of Wannier type. The single-particle bandgap is estimated to be 6.08±0.015 eV. The findings provide a comprehensive understanding of hBN's optoelectronic properties and highlight the need for further theoretical explanations of the efficient exciton-phonon interaction in hBN.Hexagonal boron nitride (hBN) is a semiconductor with unique electronic properties, including a wide bandgap, low dielectric constant, high thermal conductivity, and chemical inertness. Despite its simple crystal structure, the nature of its bandgap remains controversial, with ab initio calculations predicting an indirect bandgap and optical measurements suggesting a direct one. This paper demonstrates that hBN has an indirect bandgap at 5.955 eV, and that its optical properties are significantly influenced by phonon-assisted transitions. Using two-photon spectroscopy, the authors reveal previously unobserved lines, including a weak doublet around 5.93 eV and a dim emission at 5.955 eV, corresponding to the indirect exciton. The study also identifies the corresponding phonon modes and measures the energy splitting between the 1s and 2p exciton states for the first time in an indirect bandgap semiconductor, estimating the exciton binding energy to be 128±15 meV, indicating that excitons in hBN are of Wannier type. The single-particle bandgap is estimated to be 6.08±0.015 eV. The findings provide a comprehensive understanding of hBN's optoelectronic properties and highlight the need for further theoretical explanations of the efficient exciton-phonon interaction in hBN.