Herein, we report the first example of NR-triggered indazole formation based on the direct two-electron reduction of 2-nitrobenzylamines to reactive nitrosobenzylamine intermediates, which spontaneously cyclize and aromatize to the desired indazoles with excellent conversions (up to >99 %, Figure ). To further demonstrate the synthetic value of this route, we not only extended it to cinnoline synthesis, but also successfully coupled an imine reductase (IRED15) to the NR-triggered indazole formation in a sequential cascade reaction. With this cascade, N-methyl-2H-indazole was accessible from cheap 2-nitrobenzaldehyde and methylamine with 62 % isolated yield. This approach not only expands the reaction repertoire of flavin-dependent nitroreductases, but also adds to the available enzyme toolbox for the challenging task of indazole synthesis via ring-closing N-N bond formation in a single enzyme-catalyzed reaction step.