Porous silicon nanowires (NWs) with homogenous lateral dimensions of 90 nm are investigated by Raman scattering experiments along isothermal pressure cycles in a diamond anvil cell. Experiments were performed at variable temperatures up to 400 °C for maximal pressures of about 30 GPa comparing directly with transformations in bulk Si and porous NWs. Scanning electron microscopy demonstrates the persistence of one-dimensional morphology after high pressure investigation. The diamond phase in porous nanowires persists upon compression up to around 20 GPa at room temperature (25 °C) and to about 14 GPa at 200 °C and 400 °C. However, the β−Sn high pressure phase is seen to coexist with the diamond phase above 12 GPa at 25 °C and above 6 GPa at 200 °C and 400 °C. The coexistence region of the two phases is found to be considerably enlarged as compared with crystal silicon at each temperature. Upon decompression from 30 GPa, nucleation to the β−Sn, followed by formation of amorphous structures, is observed for porous NWs. Returning to ambient pressure and temperature, amorphous silicon is the dominant form with a residual contribution of β−Sn. At higher temperatures, nucleation back to the diamond structure is triggered although coexistence of amorphous and crystalline phases is observed up to 400 °C.