The 12-membered ring in the title compound, C20H29N3O5S.CH2Cl2, has conformational disorder with the two C atoms adjacent to the fused benzene ring each occupying two sites. The major conformer is present in 74.3(8)% of the molecules. The formal N2=C bond in the 12-membered ring and the adjacent C-N bond of the dimethylamino substituent have almost identical lengths, indicative of the electron donor character of the dimethylamino group. The amide group has the trans conformation and forms an intramolecular hydrogen bond with one of the sulfonyl O atoms. The solvent molecule forms C-H...O hydrogen bonds with the organic substrate. This novel heterocycle has been formed by a ring enlargement reaction of the corresponding nine-membered 1,2-benzothiazonin-3-one 1,1-dioxide and 2-(dimethylamino)-1-azaspiro[2.3]hex-1-ene.