A mercury-arc valve or mercury-vapor rectifier or (UK) mercury-arc rectifier is a type of electrical rectifier used for converting high-voltage or high-current alternating current (AC) into direct current (DC). It is a type of cold cathode gas-filled tube, but is unusual in that the cathode, instead of being solid, is made from a pool of liquid mercury and is therefore self-restoring. As a result mercury-arc valves, when used as intended, are far more robust and durable and can carry much higher currents than most other types of gas discharge tube. Some examples have been in continuous service, rectifying 50-ampere currents, for decades.

The New Haven EP-5 was a double-ended mercury arc rectifier electric locomotive built in 1955 by General Electric, for the New York, New Haven and Hartford Railroad. It was built to haul passenger trains between Grand Central Terminal or Penn Station in New York City and New Haven, Connecticut. The EP-5s resembled the Alco FA.