Posadnik (Посадник in Russian) was a deputy of Kniaz in some East Slavic places assigned to rule a city or a land. Most notably, the term is known as an equivalent of a stadtholder of the medieval towns of Novgorod and Pskov.

The term posadnik was first used in the Primary Chronicle under the year of 997 to denote the most senior official of an Eastern Slavic town. Later on, this term would come to mean the highest government post in Novgorod and Pskov. After the annexation of these two cities by the Muscovy, the post of posadnik was abolished.

In Novgorod Republic, the city posadnik was electable out of boyars by veche to be a mediator between the public and the Novgorod kniaz. In 1354, a statesman Ontsifor Lukinich carried out a reform, introducing six posadniks instead of one. It meant that this title was retained by the person after his replacement (hence, the term "old posadnik", or старый посадник). They also elected the so-called "stepennyi" posadnik (степенный посадник) from among the "old" ones. In accordance with the reform of 1416-1417, the number of posadniks was increased threefold and stepennyi posadniks were to be elected for a six-month period.

There were 78 known posadniks in Pskov between 1308 and 1510.