We report the formation of PtSi nanowires, PtSi/Si/PtSi nanowire heterostructures, and nanodevices from such heterostructures. Scanning electron microscopy studies show that silicon nanowires can be converted into PtSi nanowires through controlled reactions between lithographically defined platinum pads and silicon nanowires. High-resolution transmission electron microscopy studies show that PtSi/Si/PtSi heteorstructure has an atomically sharp interface with epitaxial relationships of Si[11̄0]//RSi and Si(111)//PtSi(101). Electrical measurements show that the pure PtSi nanowires have low resistivities ∼28.6 μΩ·cm and high breakdown current densities >1 × 10 8 A/cm 2. Furthermore, using single crystal PtSi/Si/PtSi nanowire heterostructures with atomically sharp interfaces, we have fabricated high-performance nanoscale field-effect transistors from intrinsic silicon nanowires, in which the source and drain contacts are defined by the metallic PtSi nanowire regions, and the gate length is defined by the Si nanowire region. Electrical measurements show nearly perfect p-channel enhancement mode transistor behavior with a normalized transconductance of 0.3 mS/μm, field-effect hole mibility of 168 cm 2/V·s, and on/off ratio >10 7, demonstrating the best performing device from intrinsic silicon nanowires.