Thermal mass (C_th, also called thermal capacitance) is the capacity of a body to store heat. It is typically measured in units of J/°C or J/K (which are equivalent). If the body consists of a homogeneous material with sufficiently known physical properties, the thermal mass is simply the amount of material present times the specific heat capacity of that material. For bodies made of many materials, the sum of heat capacities for their pure components may be used in the calculation, or in some cases (as for a whole animal, for example) the number may simply be measured for the entire body in question, directly.

As an extensive property, heat capacity is characteristic of an object; its corresponding bulk property is specific heat capacity, expressed in terms of a mass or number of moles or some other measure of the amount of material, which must be multiplied by similar units to give the heat capacity of the entire body of material. Thus the heat capacity can be equivalently calculated as the product of the mass m of the body and the specific heat capacity c for the material, or the product of the number of moles of molecules present n and the molar specific heat capacity . For discussion of why the heat storage abilities of pure substances vary, see factors that affect specific heat capacity.

Thermal mass as a concept is most frequently applied in the field of building design. In this context, thermal mass provides "inertia" against temperature fluctuations, sometimes known as the thermal flywheel effect. For example, when outside temperatures are fluctuating throughout the day, a large thermal mass within the insulated portion of a house can serve to "flatten out" the daily temperature fluctuations, since the thermal mass will absorb heat when the surroundings are hotter than the mass, and give heat back when the surroundings are cooler. This is distinct from a material's insulative value, which reduces a building's thermal conductivity, allowing it to be heated or cooled relatively separate from the outside, or even just retain the occupants' body heat longer.

Thermal mass may also be used for bodies of water, machines or machine parts, living things, or any other structure or body in engineering or biology.