The electrical resistance, R, of an array of 30 palladium nanowires is used to detect the concentration of dissolved hydrogen gas (H(2)) in transformer oil over the temperature range from 21 to 70 °C. The palladium nanowire array (PdNWA), consisting of Pd nanowires ∼100 nm (width), ∼20 nm (height), and 100 μm (length), was prepared using the lithographically patterned nanowire electrodeposition (LPNE) method. The R of the PdNWA increased by up to 8% upon exposure to dissolved H(2) at concentrations above 1.0 ppm and up to 2940 ppm at 21 °C. The measured limit-of-detection for dissolved H(2) was 1.0 ppm at 21 °C and 1.6 ppm at 70 °C. The increase in resistance induced by exposure to H(2) was linear with [H(2)](oil)(1/2) across this concentration range. A PdNWA sensor operating in flowing transformer oil has functioned continuously for 150 days.