Thermal neutron scattering cross-section measurements of heavy water (D2O) under ambient conditions were performed, for the first time, using a triple-axis spectrometer at the NRU reactor. The total cross section (σtot), as well as single (dσ/dΩ) and double differential (d2σ/dΩdE) scattering cross-sections were measured. Incident neutron energies from 15 meV to 50 meV, and scattering angles from 10° to 110° were covered. The experimental techniques and a new data analysis method based on Monte Carlo simulation to obtain the absolute cross sections are discussed, and the resulting cross-sections are compared with the Evaluated Nuclear Data File (ENDF/B-VII). A discrepancy between the existing data and the ENDF/B-VII evaluation was confirmed and resolved by a new model that was developed recently.