Rotationally resolved vibrational spectra of the three lowest frequency bands of the four-membered heterocycle azetidine (c-C 3 H 6 NH) have been collected with a resolution of 0.00096cm −1 using the far infrared beamline at the Canadian Light Source synchrotron. The modes observed correspond principally to motions best described as: β-CH 2 rock (ν 14 ) at 736.701310(7)cm −1 , ring deformation (ν 15 ) at 648.116041(8)cm −1 , and the ring puckering mode (ν 16 ) at 207.727053(9)cm −1 . A global fit of 14276 rovibrational transitions from the three bands provided an accurate set of ground state spectroscopic constants as well as excited state parameters for each of the three vibrational modes. The ground state structure was determined to be that of the puckered conformer having the NH bond in an equatorial arrangement.