Ag₂Se nanocrystals were demonstrated to be novel semiconductor mediators, or in other word catalysts, for the growth of semiconductor heterostructures in solution. This is a result of the unique feature of Ag₂Se as a fast ion conductor, allowing foreign cations to dissolve and then to heterogrow the second phase. Using Ag₂Se nanocrystals as catalysts, dimeric metal selenide heterostructures such as Ag₂S_e–CdSe and Ag₂S_e–ZnSe, and even multi-segment heterostructures such as Ag₂S_e–CdS_e–ZnSe and Ag₂S_e–ZnS_e–CdSe, were successfully synthesized. Several interesting features were found in the Ag₂Se based heterogrowth. At the initial stage of heterogrowth, a layer of the second phase forms on the surface of an Ag₂Se nanosphere, with a curved junction interface between the two phases. With further growth of the second phase, the Ag₂Se nanosphere tends to flatten the junction surface by modifying its shape from sphere to hemisphere in order to minimize the conjunct area and thus the interfacial energy. Notably, the crystallographic relationship of the two phases in the heterostructure varies with the lattice parameters of the second phase, in order to reduce the lattice mismatch at the interface. Furthermore, a small lattice mismatch at the interface results in a straight rod-like second phase, while a large lattice mismatch would induce a tortuous product. The reported results may provide a new route for developing novel selenide semiconductor heterostructures which are potentially applicable in optoelectronic, biomedical, photovoltaic and catalytic fields.