The six-year Truce of Altmark (or Treaty of Stary Targ) was signed on 25 September 1629 at the Altmark (Stary Targ), near Danzig (Gdańsk) by Sweden and the Polish-Lithuanian Commonwealth during Thirty Years' War, ending the Polish–Swedish War (1626–1629).
The conditions of the truce allowed Sweden to retain control of Livonia and the mouth of the Vistula river. Sweden also evacuated most of the Duchy of Prussia, but kept the coastal cities. the Polish-Lithuanian Commonwealth got back other Swedish gains since the 1625 invasion. The greater part of Livonia north of the Daugava River was ceded to Sweden (Swedish Livonia), though Latgale, the southeastern area, remained under Commonwealth rule. Sweden received the right to 2/3 of all the shipping tolls at ports of Polish-Lithuanian Commonwealth, such as Danzig and Elbląg (Elbing) and from the Duchy of Prussia ports for the next six years. These shipping tolls financed Sweden's involvement in the Thirty Years' War.
The Truce of Altmark was signed shortly after Sweden was defeated by Poland led by Field Crown Hetman Stanisław Koniecpolski and Holy Roman Imperial troops at Trzciana (Honigfelde) also known as Sztum (Stuhm), at which King Gustavus Adolphus of Sweden narrowly escaped capture. Gustavus was wounded several times and at one point was saved by one of his men.
The Polish parliament (so-called Sejm) did not impose new taxes in order to pay the soldiers of the imperial army fighting under Hans Georg von Arnim-Boitzenburg and due to low morale some of them mutineered or went over to the Swedish side. Several other countries intervened diplomatically and Sigismund III of Poland was eventually forced to enter the truce.
In 1635, the truce was extended via the Treaty of Stuhmsdorf. Sweden gave up the Prussian ports and the Polish-Lithuanian Commonwealth ceded most of Livonia with Riga, keeping the Daugavpils area.