A commercially available and stable ruthenium dodecacarbonyl catalyst (Ru-3(CO)(12)) allows very efficient and convenient access to functionalized silsesquioxanes (SQs) containing siloxane moiety (Si-O-Si) via dehydrogenative coupling of POSS-silanols with hydrosilanes. With the aid of SiH-containing silsesquioxanes, an unprecedented one-pot procedure has been revealed, and the usefulness of this approach was demonstrated by the synthesis of various derivatives via O-silylation, as well as C=C and C=N hydrosilylation.