The microwave spectrum of 2,3-dihydrothiophene CHCHSCH2CH2 has been recorded from 18.0 to 39.0 GHz and only a-type transitions were observed. The R-branch assignments have been made for the ground and first excited states of the ring puckering mode. The ground state rotational constants are: A = 6863.14±0.07, B = 4707.75±0.01, and C = 2953.43±0.01 MHz. It is shown that the quantity Ia+Ib−Ic = 9.87 U ⋅Å2 cannot arise totally from the four hydrogens on the two CH2 groups, but that the heavy atom skeleton contributes to it which indicates that the heavy atom skeleton is nonplanar. The rotational constants are shown to be consistent with reasonably assumed heavy atom structural parameters and a dihedral angle of puckering of 33°. The dipole moment components could not be accurately determined because of the near degeneracy of the ground and first excited state of the ring puckering mode but they were estimated to be ‖μa‖ = 1.5±0.2 D with ‖μb‖ and ‖μc‖ being relatively small. From relative intensity measurements, it is concluded that the second and third excited states are closely spaced with frequencies of 155±15 cm−1, whereas, the first excited state is nearly degenerate with the ground state. The origin of the difference band series associated with the ring puckering mode and the carbon–hydrogen stretch has been reassigned. Additionally,the first two ring puckering transitions 3←0, 2←1 have been observed in the far infrared spectrum of the gas at 167 and 152 cm−1, respectively. All of these results are consistent with a puckered ring governed by the following potential function: V(X) = 6.50×105X4−2.92×104X2, with a barrier to inversion of 328 cm−1. These results are compared to the corresponding quantities in similar molecules.