Biotransformation of 6-methylthiochroman-4-one (1) and 6-chlorothiochroman-4-one (2) was performed using Trichoderma viride in order to obtain new derivatives with antifungal properties against the phytopathogen Botrytis cinerea. Two thiochromanone derivatives are described for the first time. Antifungal activity of these compounds was tested against two different strains of Botrytis cinerea; 1 and 2 gave 100% inhibition of Bc2100 at 100-250 μg/mL, and 3 gave a maximal inhibition of 96% of BcUCA992 at 200 μg/mL. The detoxification mechanism of 1 and 2 by B. cinerea was also investigated.