The rotational spectrum of 3-pentyn-1-ol, CH 3 −C≡C−CH 2 CH 2 OH, was measured using a molecular beam Fourier transform microwave spectrometer operating in the frequency range from 2 to 26.5 GHz. A two-dimensional potential energy surface was calculated at the MP2/6-311++G(d,p) level of theory for a conformational analysis, yielding five conformers. The most stable conformer exhibits C 1 symmetry and was assigned in the spectrum by comparison with the results from quantum chemical calculations. The barrier to internal rotation of the propynyl methyl group CH 3 −C≡C− was found to be only 9.4552(94) cm −1 . Molecular parameters and internal rotation parameters could be accurately determined using the program xiam and belgi-C 1 . The internal rotation barrier was compared with those of other molecules containing a propynyl methyl group.