A visible light photocatalytic methylsulfonylation/bicyclization of C(sp³)-tethered 1,7-enynes has been established using a dimethyl sulfoxide (DMSO)/H₂O system as the methylsulfonyl source. This reaction worked readily to access a wide range of sulfone-containing benzo[a]fluoren-5-ones with one quaternary carbon centre and generally good yields through C–S bond cleavage of DMSO. The reaction mechanism was proposed based on the control experiments.