Nucleophilic substitution of bromine in 2-bromomethyl 1,3-thiaselenole with sodium azide occurs at room temperature via an intermediate seleniranium cation with the ring expansion rearrangement to six-membered 2-azido-2,3-dihydro-1,4-thiaselenine in 95% yield. The cycloaddition click reaction of the latter with various terminal acetylenes catalyzed with Cu(OAc)2 in the presence of sodium ascorbate (CuAAC) proceeds regioselectively with the formation of only 1-(2,3-dihydro-1,4-thiaselenin-2-yl)-4-organyl-1H-1,2,3-triazoles in up to 97% yields. In the case of dipropargyl sulfide, the click cycloaddition with both triple bonds is realized at room temperature affording the corresponding bis-adduct in 73% yield. The reaction of 2-azido-2,3-dihydro-1,4-thiaselenine with dimethyl and diethyl acetylenedicarboxylates gives, via the 1,3-dipolar Huisgen cycloaddition, the corresponding 1,4,5-trisubstituted 1,2,3-triazoles, functionalized with the 2,3-dihydro-1,4-thiaselenine ring, in high yields. The obtained products are potentially bioactive compounds and first representatives of 1,2,3-triazoles combined with selenium/sulfur-containing heterocycles.