Czersk (Polish pronunciation: [t͡ʂɛrsk] ⓘ) is a settlement in the administrative district of Gmina Góra Kalwaria, within Piaseczno County, Masovian Voivodeship, in east-central Poland.

Czersk was an important settlement in the past and is one of the oldest Mazovian cities, famous for ruins of a medieval castle constructed in the late 14th century.

After incorporation of Mazovia into Poland (1526), the duchy was turned into the Czersk Land (see ziemia), part of Masovian Voivodeship (1526–1795).

Czersk was first mentioned in the Laurentian Codex, in which under the date 1142, it is spelled as Чьрньскъ (Čьrnьsk, Cz(i)rn(i)sk).

In the 12th century, Czersk became the seat of a castellan, and in 1252, of a Roman Catholic archdeacon, subjected to the Bishop of Poznań, which turned the town into an important political center of Mazovia.

Officially, Czersk received town charter in 1350, but long before that time, it had an urban character, with a mid-13th century market square.

The town never recovered from the destruction; in 1762-1766 Crown Marshall Franciszek Bieliński tried to rebuild the castle, but his project was not completed due to Partitions of Poland.

In the 11th century, during the reign of either Bolesław III Wrymouth or Władysław I Herman, a wooden gord existed in the location of the castle.