A number of 4-amino-5-aryl-2,3-dihydro-3H-1,2,4-triazole-3-thiones were synthesized and tested for inhibitory activity against trans-cinnamate 4-hydroxylase (C4H) from Populus kitakamiensis, which was expressed in yeast. Of the compounds tested, 4-amino-5-phenyl-2,4-dihydro-3H-1,2,4-triazole-3-thione (1) was the most effective, having an IC50 value of 0.32 mM. Replacement of the phenyl group with heterocycles or substituted phenyl groups drastically reduced the activity. Compound 1 behaved as a mixed-inhibitor of C4H. When potato tuber disks were treated with 1 (0.2–20 ng/disk) in the presence of laminarin, a β-1,3-glucooligosaccharide elicitor, trans-cinnamic acid was accumulated at levels 3 times higher than in disks treated with laminarin alone.