A bain-marie (or "water bath"; plural bains-marie) is an apparatus used in cooking for applying gentle heat to food. The food is placed in a container which in turn is placed in a shallow dish filled with hot water, which is then usually placed in an oven. 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, the French bain de Marie meaning "Mary's bath".

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.

A similar device is a double boiler.