In chemistry, Markovnikov's rule is an observation based on Zaitsev's Rule. It was formulated by the Russian chemist Vladimir Vasilevich Markovnikov . It states that, in chemical reactions found particularly in organic chemistry, when a hydrogen halide reacts with the carbon-carbon double bond of an unsymmetrical alkene, giving an alkyl halide, the hydrogen adds to the carbon of the alkene functional group that has the greater number of hydrogen substituents, and the halogen adds to the carbon on the other end of the double bond which has a smaller number of hydrogen substituents. Therefore when a molecule of the form HX (where X is more electronegative than H) is added in an addition reaction to a carbon-carbon double bond, the H is added to the less substituted carbon atom, while the X is added to the more substituted. The stability of the carbocation reactive intermediate directly causes this rule.

The rule may be summed up by quoting that the rich get richer and the poor get poorer, in that a carbon rich in substituents will get more substituents and the carbon with more hydrogens attached will get the hydrogen in case of many different organic addition reactions.

Mechanisms which avoid the carbocation intermediate may react through other mechanisms that are regioselective, against what Markovnikov's rule predicts: such as free radical addition. Such reactions are said to be anti-Markovnikov, since the halogen adds to the least substituted carbon.