AbstractAn accurate semi‐experimental equilibrium structure of 8‐hydroxyquinoline (8‐HQ) has been determined combining experiment and theory. The cm‐wave rotational spectrum of 8‐HQ was recorded in a pulsed supersonic jet using broadband dual‐path reflection and narrowband Fabry‐Perot‐type resonator Fourier‐transform microwave spectrometers. Accurate rotational and quartic centrifugal distortion constants and 14N quadrupole coupling constants are determined. Rotational constants of all 13C, 18O and 15N singly substituted isotopologues in natural abundance and those of a chemically synthesized OD isotopologue were used to obtain geometric parameters for all the heavy atoms and the hydroxyl hydrogen from a number of structure determination models. Theoretical approaches allowed for the determination of a semi‐experimental equilibrium structure, in which computed rovibrational and electronic corrections were utilized to convert vibrational ground state constants into equilibrium constants. Despite the molecule having only a horizontal plane of symmetry and possessing 11 individual heavy atoms, microwave spectroscopy has allowed for a reliable and accurate structure determination. A mass dependent, structure was determined and proved to be equally reliable by comparison with the B3LYP‐D3(BJ)/aVTZ equilibrium structure.