Ultrathin SnO2 nanosheets were prepared by a hydrothermal method using SnF2 and methenamine as precursor and morphology controlling agent, respectively. Structural characterizations indicate that these ultrathin SnO2 nanosheets having a thickness of approximately 4-6 nm can assemble into a three-dimensional, flowerlike architecture. Due to the higher electron mobility and enhanced light-scattering effect of these hierarchical structures, the dye-sensitized solar cells (DSSCs) based on such SnO2 architectures exhibit much higher cell performance than that of SnO2 nanoparticles. Furthermore, coating a TiO2 layer on these ultrathin SnO2 nanosheets can also significantly improve the short-circuit current, open-circuit voltage, and fill factor. Compared with the plain SnO2 nanosheets, the TiO2 coating on these ultrathin SnO2 nanosheets can lead to more than 7 times improvement in the energy conversion efficiency. With a thin layer of TiO2 coating, the highest overall photoconversion efficiency of DSSCs based on SnO2 nanosheets is approximately 2.82%, which is over 2 times higher than that of DSSCs constructed by conversional SnO2 nanoparticles.