[3]Cyclo-1,8-carbazolylene was found to undergo oxidative rearrangement by oxidation with 2,3-dichloro-5,6-dicyano-p-benzoquinone, giving a 9a-substituted carbazol-l-one derivative. This is the first example of a dearomative oxidative rearrange-ment of a carbazole or aniline derivative. The geometrical and electronic structures were elucidated by X-ray analysis, UV-vis absorption spectroscopy, and DFT calculations.