Excitonic band structure is critical for investigating exciton dynamics. Theoretically, quantum effects from exchange scattering between electron-hole pairs significantly modulate exciton dispersion. Here, we report the direct observation of dimensionality-dependent exciton dispersion in a single-band Mott insulator ${\mathrm{Nb}}_3{\mathrm{Cl}}_8$ through high-resolution electron energy loss spectroscopy. In the high-temperature phase, the exciton in ${\mathrm{Nb}}_3{\mathrm{Cl}}_8$ hosts an exceptionally large binding energy, and exhibits clear quasi-two-dimensional massless linear dispersion. In contrast, in the low-temperature phase, the exciton splits into two bands, both displaying three-dimensional parabolic dispersion. These dramatic changes in the exciton dispersion stem from the dimensional mutation driven by a substantial enhancement of interlayer coupling across the phase transition. This Letter provides a clear and typical example of how exciton behavior evolves with dimensionality.

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• Excitons
• Structural phase transition
• 2-dimensional systems
• Mott insulators
• Semiconductor compounds
• Electron energy loss spectroscopy