A huge positive magnetoresistance (MR) effect, about 4 orders of magnitude at room temperature, has been discovered in metal/semiconductor hybrid nanostructures. The hybrid film consisting of metallic nanostructures that are fabricated on a GaAs substrate by ultra-high vacuum deposition method exhibits magnetic-field-sensitive current-voltage characteristics. When a constant voltage, above the threshold value, is applied to the film, very steep change in the current, which we term magnetoresistive switch, is driven by the huge MR effect under a relatively low magnetic field at room temperature. The MR effect is very sensitive to the nanoscale morphology of the hybrid film, in other words, one can control the MR function at will by modifying the nanostructure in this smart system. In this paper, the origin of the magnetoresistive switch effect and the possible application as a magnetic field sensor will be discussed.