An unprecedented [4 + 2] annulation reaction between in situ formed azoalkenes and azlactones has been developed. This reaction provides a facile access to an array of 4,5dihydropyridazin-3(2H)-one derivatives, which are very promising in medicinal applications as potential biologically active candidates. Notably, these dihydropyridazinones could also be synthesized via a one-pot reaction protocol by using the in situ formed azlactones from N-acyl amino acids and in situ generated azoalkenes from αhalogeno hydrazones. The potential applications of the methodology were also demonstrated by gram-scale experiments and the versatile conversions of the products into other nitrogen-containing compounds.