The mole (symbol mol) is the unit of amount of substance in the International System of Units (SI). The quantity amount of substance is a measure of how many elementary entities of a given substance are in an object or sample.

The mole is defined as exactly 6.02214076×10²³ elementary entities. Depending on the nature of the substance, an elementary entity may be an atom, a molecule, an ion, an ion pair, or a subatomic particle such as a proton. For example, 10 moles of water (a chemical compound) and 10 moles of mercury (a chemical element) contain equal amounts of substance, and the mercury contains exactly one atom for each molecule of the water, despite the two substances having different volumes and different masses. The number of elementary entities in one mole is the Avogadro number, which is the approximate number of nucleons (protons or neutrons) in one gram of ordinary matter.

Before the 2019 redefinition of SI base units, the mole was defined as the number of elementary entities in 12 grams of carbon-12 (the most common isotope of carbon).

The mole is widely used in chemistry as a convenient way to express amounts of reactants and amounts of products of chemical reactions. For example, the chemical equation 2H₂ + O₂ → 2H₂O can be interpreted to mean that for each 2 mol dihydrogen (H₂) and 1 mol dioxygen (O₂) that react, 2 mol of water (H₂O) form. The concentration of a solution is commonly expressed by its molar concentration, defined as the amount of dissolved substance per unit volume of solution, for which the unit typically used is moles per litre (mol/L).

The term gram-molecule was formerly used to mean one mole of molecules, and gram-atom for one mole of atoms. For example, 1 mole of MgBr₂ is 1 gram-molecule of MgBr₂ but 3 gram-atoms of MgBr₂.