A bain marie, also known as a double-boiler, is a type of pot used to heat substances to a controlled temperature. It is useful in applications in which overheating must be prevented to yield desirable results. The bain marie is used mainly in chemistry and cooking, particularly candymaking, but many other industries use the bain marie to manufacture their products as well.

A bain marie consists of a smaller pot inside a larger one. The larger pot is filled with a liquid, typically water, while the substance to be heated goes in the smaller pot. The bain marie prevents the material in the smaller pot from rising above the boiling point of the liquid in the larger pot. The maximum temperature of the smaller pot can be adjusted by changing the liquid in the larger pot; adding salt to water will result in a higher boiling point, for example, while adding alcohol will lower the boiling point.

A typical use for the bain marie in cooking is melting chocolate. Since the boiling point of water is 212° Fahrenheit (100° Celsius), the chocolate in the bain marie cannot heat above this temperature. Melting chocolate over a direct flame would be very difficult and would most likely result in a burnt, unusable product. In chemistry, the results of using a regular pot when a bain marie is indicated could be even more disastrous. A bain marie also helps the substance in the smaller pot heat at a more even temperature than would be possible over a direct flame.

A bain marie is a must-have for the gourmet kitchen. Besides melting chocolate, it is used to make cheesecake, custard, and several other milk-based sweets. In addition, some consider the bain marie the ideal way to warm up breast milk for a baby.

The bain marie was first used in alchemy, the ancient forerunner of today's chemistry. Its name means roughly "Mary's bath," as legend tells that it was invented by a female alchemist named Mary, traditionally assumed to be Miriam, the sister of the biblical Moses.