A spherosilicate dendrimer (DMS-1) with closely spaced reaction sites (SiH groups) on the dendrimer surface has been synthesized by stepwise silylation of double-four-ring silicate with chlorotriethoxysilane (ClSi(OEt)3) and subsequently with chlorodimethylsilane (ClSiHMe2). DMS-1 consists of a maximum of 40 Si atoms in the interior frameworks and 24 reactive SiH groups on the surface. Because DMS-1 is spherical and about 1.5 nm in diameter, it can be regarded as the smallest well-defined silica-based nanoparticle. DMS-1 also forms molecular crystals and is soluble in typical organic solvents. A molecularly ordered silica-based hybrid can be prepared by heating a cast film of DMS-1 at 180 °C for 5 days. The surface of DMS-1 can be modified by hydrosilylation with 1-hexadecene, triethoxyvinylsilane, and allylic-terminated tetraethylene glycol monomethyl ether. More than 20 SiH groups out of 24 react with these reagents. The solubilities of the products depend on the modification. DMS-1 is not only a building block for nanohybrids, but also the smallest and most precisely designed siloxane-based nanoparticle.