Bain-marie or Mary's bath is a method utilized in the kitchen to slowly warm or convey uniform temperature to a liquid or solid substance, by submerging the container into a larger one with boiling or near boiling water.

The main concept here is that of bath which implies indirect heating of a substance. Heat transfer to the medium (water of the bath) is by convection, while conduction is the process involved in the warming of the substance. The medium ( bath ) could be mineral oil; pure water, salt solutions of varying solutes concentrations, etc., depending on the temperature at which the substance is required to be warmed. It is customary to add medium at ambient (room) temperature to decrease its temperature and consequently the heat transfer rate from it to the substance.

The term originates from alchemy, where some practitioners needed to give their materials prolonged periods of gentle heating, in an attempt to mimic the supposed natural processes whereby precious metals germinated in the earth. It was said to be an invention of Mary the Jewess, an ancient alchemist and traditionally supposed to have been Miriam, a sister of Moses. The name comes from this tradition: balneum Mariae in medieval Latin, from which the French bain de Marie is derived, although, in the french wikipedia the coinage of the term is attributed to the medieval German philosopher and theologian Saint Albert the Great (1193-1280).

Bain-marie (or "water bath"; plural bains-marie ) is also the apparatus used in the warming process. A similar device is a double boiler.

• Cheesecake is generally baked in a bain-marie to prevent the top from cracking in the center.
• Custard, for example, may be cooked in a bain-marie to prevent a crust from forming on the outside of the custard before the interior is cooked.
• Melting of chocolate for use in topping of various dishes
• Dehydration and thickening of condensed milk to make several milk-sweets
• Controlled-temperature bain-marie is recommended to lukewarm freeze-stored breast milk when timely breastfeeding is not possible, according to breastfeeding advocates.