Copyright © 2019 American Chemical Society. Tin oxide quantum dots (QDs) have attracted much attention because of their low toxicity and the absence of cadmium and other poisonous elements. In this paper, we report the novel synthetic method for size-controlled tin oxide QDs using dendrimers as a template, and their electronic and structural properties. Hemispherical tin oxide QDs with a size below 2 nm and small size distribution were synthesized on silica glass substrates by the dendrimer-templated synthesis method (Sn12, Sn28, and Sn60 oxide QDs). The structures of the tin oxide QDs were composed not only of Sn(IV) sites, but also Sn(II) sites due to the restriction of the coordination environment to stabilize the structure. Density functional theory calculation showed that a bare tin oxide cluster with a mixed valence state (Sn(II) + Sn(IV)) is more stable than those only with Sn(II) or Sn(IV). The synthesized tin oxide QDs showed the quantum confinement effect caused by the spatial confinement of an electron. The Urbach tail parameter, expressing the disorderliness of the QDs, decreased with the reduced QD size, although the value of each tin oxide QD was higher than that of bulk SnO2. The experimental band gap energy was compared with the effective mass approximation models, which are theoretical models for the quantum confinement effect. We found that the experimental values of Sn28 and Sn60 oxide QDs were consistent with the theoretical values, while Sn12 oxide QDs had a lower value compared to the predicted band gap energy. This could be attributed to the change in the physical parameters of Sn12 oxide QDs, which are not the same as those of Sn28, Sn60 oxide QDs or the bulk SnO2. These results indicate that small tin oxide QDs have a different structure and different electronic properties compared to bulk or conventional nanoparticles and have potential applications in such fields as catalysis and optical and electronic devices.