Peter Cooper Hewitt (May 5, 1861 - August 25, 1921) was an American electrical engineer, who demonstrated the mercury-vapor lamp for which he deposited a patent.

He fabricated a discharge lamp in a vapor of mercury heated by the current passing through the liquid phase. The lamp was started by tilting the tube to make contact between the two electrodes, with the liquid mercury located on one side at rest. The efficiency was much higher than incandescent lamps but the emitted light was of a bluish-green unpleasant color, which limited its practical use to specific professional areas, like photography where the color was not an issue at a time where films were black and white.

In 1902 Hewitt developed the first mercury arc rectifier, which were an efficient way of converting alternating current power to direct current for use in electric railways, industry, and HVDC power transmission.