The title compound, C5H9BrN4O, was obtained as a minor by-product in the synthesis of 4-dimethylamino-1-methyl-1,2,4-triazolin-5-one. Except for the methyl groups of the 4-dimethylamino moiety, all the non-H atoms lie on a crystallographic mirror plane." In the crystal, the molecules are linked by C—Br⋯O=C interactions [Br⋯O = 2.877 (2) Å, C—Br⋯O = 174.6 (1)°] into infinite chains in the c-axis direction.