The rotational spectrum of benzyl mercaptan (parent and four isotopologues) has been assigned in a supersonic jet expansion using chirped-pulse Fourier transform microwave spectroscopy. The spectrum is characterized by torsional tunneling doublings, strongly perturbed by Coriolis interactions. The experimental rotational constants reveal that the sulfur atom is located above the ring plane in a gauche conformation. The torsion dihedral theta(0) = phi (SC alpha-C1C2) is approximately 74 degrees, according to a flexible molecular model calculation reproducing the energy separation (Delta E-01 similar to 2180.4 MHz) between the first two torsional substates. The global minimum configuration is 4-fold degenerate, corresponding to potential minima with theta(0) +/- 74 degrees and +/-(180-74)degrees. The four equivalent minima are separated by potential barriers at theta = +/- 90 degrees, 0 degrees, or 180 degrees. The tunneling splittings are caused by the potential barrier at theta = +/- 90 degrees, while the barriers at torsions of 0 degrees or 180 degrees are too large to generate detectable splittings. The tunnelling barrier has been determined as 248 cm(-1), similar to the value obtained with high-level MP2 ab initio calculations (259 cm(-1)), but smaller than in benzyl alcohol (280 cm(-1)).