A novel tree-like nanostructured Ag crystal, with stems, branches, and leaves, has been synthesized by pre-forming Au seeds, soaking, and annealing, based on monolithic mesoporous silica. The obtained Ag nanotrees are of single-crystal nature and statistically symmetrical in geometry. Further experiments revealed that the interconnected channels of the porous silica, heating at low temperature, and the pre-formed Au seeds are crucial to form such structure. Its formation can be attributed to the low nucleation rate and preferentially unidirectional diffusion of Ag atoms to the Au seeds along interconnected channels. This nanostructured material is of great potential to be building blocks for assembling some mini-functional devices of the next generation. The current study is also of importance in studying the diffusion mechanism of single-crystal formation, and especially in improving our understanding of the underlying physical structure of both natural and synthetic porous materials.