Cinnoline derivatives are important nitrogen-containing heterocycles widely found in biologically active molecules and pharmaceutical compounds. Herein we report an efficient synthetic approach to cinnoline derivatives through C–H/N–H annulation of N-aryl triazolinediones with diazo compounds. Ir(III) catalysis with diazo Meldrum’s acids affords dihydrocinnolinones, whereas Ru(II) catalysis with α-diazo-β-ketoesters provides cinnoline derivatives through an annulation and dehydration sequence. This method enables the efficient construction of diverse cinnoline frameworks with good functional-group tolerance and synthetic utility, offering a useful strategy for accessing pharmaceutically relevant heterocycles.