The microwave spectra of methyl n-propyl sulfide, CH3-S-CH2-CH2-CH3, were recorded in the frequency region from 2.0 to 26.5 GHz, revealing three conformers. Quantum chemical calculations were carried out to support experimental work. Fine splittings arising from the internal rotation of the methyl group attached to the sulfur atom were resolved and analyzed. Torsional barriers of about 600 cm-1 for the two conformers with C1 symmetry and about 700 cm-1 for the Cs conformer could be deduced, showing that conformations affect the methyl internal rotation. Torsional splittings of the methyl group at the end of the propyl moiety were observed for some transitions, leading to the determination of barrier heights close to 1000 cm-1. The spectrum of the 34S isotopologue of the most stable conformer could be measured in natural abundance. The present laboratory work provides highly accurate spectroscopic parameters, which serve as reliable starting values for extrapolation in higher frequency ranges and for the search of this sulfur-containing molecule in the interstellar medium.