Abstract: A hand warmer device maintains hand warmth while not inhibiting dexterity in the wearer. The device includes a flexible sheet having an adhesive portion on an inner face thereof. The sheet is formed generally in the shape of a hand in two dimensions. Affixed to an outer face of the sheet is an activatable heating element containing a composition including materials the mixing of which causes an exothermic reaction. In use, a person can affix the sheet to a top of the hand by adhering the inner face thereto. The heating element can be activated by flexing the fingers, causing a mixing of the composition materials, and thereby the emission of heat. The heating device can thus keep the hand and fingers warm for some number of hours while leaving the palms and insides of the fingers free to perform a desired task.