The direct formation of multisegment nanowires consisting of polymer domains by ion beam irradiation is investigated. Cross-linking reactions in the ion tracks result in localized gelation, giving isolated nanowires on substrates. It is demonstrated that the morphology of the final nanostructure is customized by appropriate selection of the ion fluence, combination of polymers, and the solvent employed for development. Octopus-like nanostructures consisting of a tangled hydrophilic polymer core and splayed hydrophobic polymer segments are successfully produced as an example of the process. The present technique provides universal feasibility for the formation of nanostructures based on "any" polymer materials in which radiation induces cross-linking reactions.