Thio amino acids can be integrated into the backbone of peptides without significantly perturbing their structure. In this contribution we use ultrafast infrared and visible spectroscopy as well as state-of-the-art ab initio computations to investigate the photoisomerization of the trans form of N-methylthioacetamide (NMTAA) as a model conformational photoswitch. Following the S-2 excitation of trans-NMTAA in water, the return of the molecule into the trans ground state and the formation of the cis isomer is observed on a dual time scale, with a fast component of 8-9 ps and a slow time constant of similar to250 ps. On both time scales the probability of isomerization to the cis form is found to be 30-40%, independently of excitation wavelength. Ab initio CASPT2//CASSCF photochemical reaction path calculations indicate that, in vacuo, the trans --> cis isomerization event takes place on the S-1 and/or T-1 triplet potential energy surfaces and is controlled by very small energy barriers, in agreement with the experimentally observed picosecond time scale. Furthermore, the calculations identify one S-2/S-1 and four nearly isoenergetic S-1/S-0 conical intersection decay channels. In line with the observed isomerization probability, only one of the S-1/S-0 conical intersections yields the cis conformation upon S-1 --> S-0 decay. A substantially equivalent excited-state relaxation results from four T-1/S-0 intersystem crossing points.