The microwave spectrum of propargyl selenocyanate (HC CCH2SeC N) has been investigated in the 40-80 GHz spectral region at 0 degrees C. The spectra of the ground vibrational state of the Se-80 and Se-78 isotopologues of one conformer were assigned. The first vibrationally excited state of the torsion about the CC-Se bond was also assigned for the Se-80 isotopologue. The identified rotamer has a C-C-Se-C antiperiplanar conformation (C-s symmetry). The microwave work has been augmented by ab initio calculations at the MP2/6-311++(3df,3pd) level and density functional theory calculations at the B3LYP/6-311G++(3df,3pd) level of theory. Both the ab initio and the B3LYP calculations predict that the identified rotamer is the global minimum and indicate the existence of an additional high-energy anticlinal form. The two theoretical methods predict rather different values for the H2C-Se bond length as well as for the C-C-Se-C dihedral angle of the high-energy anticlinal form.