A convenient route to prepare unsubstituted pyridazine‐3,4‐dicarboxylic acid on a preparative scale is described. The synthesis involves a hetero Diels‐Alder reaction between a new 1,2‐diaza‐1,3‐diene and ethyl vinyl ether and oxidation of the intermediate 1,4,5,6‐tetrahydropyridazine as the key step.