The Lady Knox Geyser is a geyser in the Wai-o-Tapu area of the Taupo Volcanic Zone in New Zealand. It is named after Lady Constance Knox, the second daughter of Uchter Knox, 15th Governor of New Zealand. The geyser is induced to erupt daily at 9:45am by dropping a surfactant into the opening of the vent. Eruptions produce a jet of water reaching up to 20m and can last for over an hour., depending on the weather. The visible spout is made of rocks placed around the base of the spring to enhance the eruption; over the years silica from the eruptions has built up to give a white cone-shaped appearance.

Because the geyser was discovered early in the 20th century, it has no Māori name, unlike almost every other thermal feature in the region.

The geyser has two water chambers, one lower, hot one and one upper, cold one. The upper chamber cools due to a larger opening to the outside. The lower one heats up due to the volcanic activity below. When soap is thrown into the upper water chamber, the lowered surface tension of the water allows it to mix with the hotter water below, causing the eruption.