A catalytic, transition-metal-free synthesis of spiroisoxazolines is outlined. This protocol provides, for the first time, a direct access to spiroisoxazolines from aryl (thio)ethers or (thio)phenols in one synthetic step via an oximation/dearomatization cascade. In this reaction, sodium nitrite plays dual roles: as a hydroxylamine source and also a precatalyst to promote the aerobic dearomatization. This methodology features unprecedented substrate scope, high yields, promising scalability, sustainable oxidant, and mild conditions.