A mild and efficient tool to construct structurally diverse N/O spiroheterocycles via Rh(III)-catalyzed [3+3] cascade spiroannulation of benzoxazines with 1-diazonaphthalen-2(1H)-ones has been developed. The desired spirooxazine-pyrans could be afforded in up to 99% yields at room temperature within 30 min. This transformation involves C–H activation, tautomerization, and intramolecular nucleophilic addition in a “one-pot” manner. In addition, this catalytic system features high efficiency, atomic economy, wide functional group tolerance and extremely mild reaction conditions.