A full exploration of the conformational landscape of D-ribose and 2-deoxy-D-ribose monosaccharides in the gas phase has been performed using DFT methods (B3LYP and M06-2X). Open-chain, furanose and pyranose configurations have been examined. Up to 954 and 668 stable structures have been obtained for D-ribose and 2-deoxy-D-ribose. Among these structures, up to 35 and 22 have relative energies smaller than 5 kJ mol−1 with respect to the absolute minimum of each molecule, respectively. For D-ribose, pyranose in α- and β-forms is the most populated according to both functionals, the β-diastereoisomer being the most populated. For 2-deoxy-D-ribose, the α-pyranose form is in majority. The β/α relationship of pyranose forms presents different results for both functionals: for M06-2X it increases in D-ribose and decreases in 2-deoxy-D-ribose at 0 K with respect to the room temperature results, the opposite case occurring in B3LYP. Intramolecular weak interactions have been characterized using the AIM and NBO methodologies.