We demonstrate that Ag2S nanocrystals are the bifunctional mediator for controllable growth of semiconductor heterostructures including more complicated multisegments heterostructures in solution-phase, which is a new type of nanomediator and quite different from the metal nanoparticle catalyst. The intrinsic high Ag+ ion mobility makes Ag2S nanocrystals not only exhibit excellent catalytic function for growth of metal sulfide heterostructures but also act as a source-host for growth of ternary semiconductor heterostructures, for example, Ag2S-AgInS2. The semiconductors grow epitaxially from or inward in Ag2S nanocrystals forming single-crystalline heterostructures. Moreover, the method developed here also can construct multisegments heterostructures, for example, Ag2S-CdS-ZnS, AgInS2-Ag2S-AgInS2. The interfacial structure is still stable even if the lattice mismatch is quite large, which is a unique feature of this method.