A catalytic silylium-ion-promoted intramolecular alkyne carbosilylation reaction is reported. The ring closure is initiated by electrophilic activation of the C-C triple bond by a silylium ion, and the catalytic cycle is then maintained by the protodesilylation of a stoichiometrically added allylsilane reagent. Exclusive 7-endo-dig selectivity is seen, leading to a series of silylated benzocycloheptene derivatives with a fully substituted vinylsilane. Control experiments showed that the catalytically active silylium ion can also be regenerated by protodesilylation of the vinylsilane product.