The formation of a hydrogen bond between methylene cyclobutane and hydrogen chloride has been investigated in the region 8-18 GHz using molecular-beam Fourier transform microwave spectroscopy. The rotational spectrum of an endo conformer in which methylene cyclobutane retains a puckered conformation and HCl is bonded in T-shape to the methylenic group has been observed. Searches for the exo form using Ar, Ne or He as carrier gases were unsuccessful. A C-s symmetry r(0)-like structure has been obtained for this conformer from the rotational data of three observed isotopomers (C5H8...(HCl)-Cl-35, C5H8...(HCl)-Cl-37, and C5H8...DCl). Ab initio calculations at MP2/6-311+G(d,p) level have been used to complement the analysis. These calculations predict the existence of both endo and exo conformers close in energy.