Understanding fundamental crystal nucleation and growth mechanisms is critical for producing materials with controlled size and morphological features and uncovering structure–function relationships in these semiconducting oxides. Under hydro-solvothermal conditions, uniform branched and spherulitic TiO2 rutile nanostructures were formed via (101) twins. On the basis of detailed, high-resolution scanning electron microscopy and transmission electron microscopy analyses, we propose a mechanism of branched growth and the (101) twin formation via oriented attachment and subsequent transformation from anatase to rutile.