In the crystal structure of 1-methyl-3-allyl benzotriazolium iodide, C(10)H(12)N(3) (+)·I(-), centrosymmetric dimers of coplanar cations are π-stacked with an inter-planar distance of 3.453 (6) Å. The iodide anions are situated above and below the formally positive charged triazolium rings.