Novel tetravinyl- and tetraallyl-substituted closed double-decker siloxanes (DDSQs) were synthesized and characterized, and their structures were elucidated by X-ray crystallographic analysis. Moreover, it was shown that peripheral olefins could successfully undergo hydrosilylation quantitatively. Such tetrafunctionalizable DDSQs (DDSQ-Vinyl4 and DDSQ-Allyl4) thus constitute promising building blocks for more complex inorganic–organic hybrid materials. Synopsis Novel tetravinyl- and tetraallyl-substituted closed double-decker siloxanes (DDSQs) were synthesized and peripheral olefins shown to successfully undergo hydrosilylation quantitatively. Such tetrafunctionalizable DDSQs thus constitute promising building blocks for more complex inorganic−organic hybrid materials.