An equation relating the structure to the reactivity of side-chain derivatives of aromatic compounds. It arises from a comparison between rate constants for various reactions with the rate of hydrolysis of benzyl chloride on the one hand and a comparison between equilibrium constants (such as the dissociation constant of benzoic acid) on the other hand. The Hammett equation can be written in the form log(k/k₀) = ρ‎log(K/K₀), where log(K/K₀) refers to comparing dissociation constants to the dissociation constant, K₀, of benzoic acid in water at 25°C, and log(k/k₀) refers to comparing rates of reaction to the rate, k₀, of hydrolysis of benzyl chloride. The term log(K/K₀) = σ‎ is called the substituent constant, since the nature of the substituent affects the strength of the benzoic acid. If σ‎ is positive, the substituent is electron attracting, while if σ‎ is negative the substituent is electron donating. ρ‎ is a reaction constant, which is determined for a given reaction by the slope of a graph of log(k/k₀) against σ‎. The numerical value of ρ‎ depends on temperature and the type of solvent.

The Hammett equation applies to meta- and para- substituents (provided that resonance interaction from the substituents does not occur) but not to ortho-substituents. It was derived by Louis Hammett (1894–1987) in 1937.