A catalyst-free electrochemical [3 + 2] cycloaddition of azobenzenes with hexahydro-1,3,5-triazines in the absence of any external oxidant is developed for constructing the 1,2,4-triazolidine skeleton; here 35 examples are displayed. The unique electrochemical cycloaddition features wide substrate scope, excellent functional group tolerance and satisfactory yields. Mechanistic experiments support the reaction pathway proposed for the [3 + 2] cycloaddition.