<p>Excitons—elementary excitations formed by bound electron–hole pairs—govern the optical properties and excited-state dynamics of materials. In two dimensions, excitons are theoretically predicted to have a linear energy–momentum relation with a non-analytic discontinuity in the long wavelength limit, which mimics the dispersion of a photon. This results in an exciton with a dispersion resembling a massless particle, despite it being a composite boson composed of massive constituents. However, direct experimental observation of massless excitons has not been achieved. Here we experimentally demonstrate the linear exciton dispersion in free-standing monolayer hBN using momentum-resolved electron energy-loss spectroscopy. The observation is consistent with our theoretical prediction based on ab initio many-body perturbation theory. Additionally, we identify the lowest dipole-allowed transition in monolayer hBN to be at 6.6 eV, which illuminates a long-standing debate about the bandgap of monolayer hBN. These findings provide critical insights into two-dimensional excitonic physics and show potential pathways for exciton-mediated superconductivity, Bose–Einstein condensation and high-efficiency optoelectronic applications.</p>