The microwave spectrum of 3-buteneselenol (H2C=CH-CH2-CH2SeH) has been investigated in the 24-62 GHz spectral range by Stark microwave spectroscopy and by Fourier transform microwave spectroscopy in the 6-20 GHz region. High-level quantum chemical calculations carried out for 14 conformers predict that their energies fall within a narrow range of less than 7 kJ/mol. The most stable conformer was predicted to have an intramolecular hydrogen bond formed between the hydrogen atom of the selenol group and the double bond. The microwave spectra of the three most stable conformers were assigned. One of these rotamers has an internal hydrogen bond, whereas the two others are without this interaction. It was found by relative intensity measurements that the hydrogen-bonded conformer is 1.6(7) and 1.9(5) kJ/mol, respectively, more stable than the two other rotamers. As far as we know, this is the first example of a gas-phase study demonstrating that a selenol indeed participates in intramolecular hydrogen bonding.