The first synthesis of the kinetically unstable compound propa-1,2-dienethiol (allenethiol; H2C=C=CHSH) is reported. Its microwave spectrum has been studied in the 41.5-80 GHz spectral range. The spectra of two rotameric forms have been assigned. The C-C-S-H chain of atoms is synperiplanar (0 degrees) in one of the conformers. This dihedral angle is anticlinal in the second rotamer forming an angle of 140(5)degrees in the second form. The synperiplanar conformer is found to be 1.0(6) kJ/mol more stable than the anticlinal rotamer. The microwave study has been augmented,by quantum chemical calculations at the MP2/aug-cc-pVTZ and B3LYP/6-311++G** levels of theory. The predictions of these two theoretical methods are in excellent agreement with the experimental findings.