It is located in the eastern part of the Suwałki Lake Area (Polish: Pojezierze Suwalskie), on the Marycha river (Seina in Lithuanian for which the town was named),[3] being a tributary of the Czarna Hańcza.

[4] The first written record of the area where the town now lies dates to 1385, noting an armed raid of the German knights from Castrum Leicze (Giżycko) to Merkinė.

After the Treaty of Melno in 1422, Teutonic-Lithuanian border was determined, people began to return to the forests in the area.

The territory formed part of the Grand Duchy of Lithuania within the Polish–Lithuanian union under the Jagiellonian dynasty, since 1569 transformed into the Polish–Lithuanian Commonwealth.

On December 22, 1522, Sigismund I the Old ordered the voivode of Podlaskie Janusz Kostewicz to grant 1.5 km2 (1⁄2 sq mi) of land at the shores of the river Sejna (now called Marycha) to hetman Iwan Wiśniowiecki.

A new wooden manor was built in the place where the Sejna river reached Sejny lake and soon settlement started.

In 1593 the town was sold by his great-granddaughter Anna, wife of voivode of Witebsk Mikołaj Sapieha, to a local noble Jerzy Grodziński for sixty times the amount of 10.000 grosz in silver.

The town's market was located on a small hill overlooking the right bank of the river, near the original wooden manor.

South of the town, a new road leading to Grodno was created and the new settlement received significant income from trading.

On November 8, 1670, the king Michael I granted the town the privilege of organizing a market and fair once a week.

In the early 18th century the Great Northern War put an end to the prosperity, as the town was pillaged by several armies in a row.

With increased prosperity in the town, the Dominicans started the reconstruction of Sejny, leading to the construction of notable examples of baroque architecture.

The church received a new façade, in 1770 a new town hall was built, and in 1778 a new marketplace and a new Wooden synagogue were opened.

Initially neglected, in 1807 the town became part of the short-lived Polish Duchy of Warsaw and a major administrative centre within the Łomża Department.

Nonetheless, the Neo-baroque Sejny Synagogue was built in the 1860s, now used as a cultural center after the deportation and murder of the Jews by Nazi Germany in the Holocaust during World War II.

Under pressure from the Conference of Ambassadors (later to become the League of Nations), Poland initially backed down on the issue, but, on August 22, 1919, on the day German troops withdrew from the area, Polish Military Organization organized a military action against the Lithuanian rule in what became known as the Sejny Uprising (or "Seinai Revolt").

[9][10][11][12][13] The Polish historian Piotr Łossowski has suggested that both sides exaggerated repressions they suffered during the uprising and its aftermath in order to elicit internal and external support.

To ensure the right of passage through Lithuanian territory, on 12 July 1920 the Soviet authorities signed the Soviet-Lithuanian Treaty of 1920, in which it ceded the area to Lithuania.

In 1925 the bishopric and the powiat status were removed, but the town remained a notable centre of not only trade and commerce but also wood and furniture production, gaining profits from the nearby forests.

During the invasion of Poland, which started World War II, in 1939 the town was first captured by the Soviet Union on September 24, 1939.

Poles were subjected to mass arrests, executions and deportations to Nazi concentration camps, and the local Jewish community was almost completely destroyed.

[17] Poles managed to organize secret teaching, but soon the Germans carried out mass arrests of Polish teachers, who were then imprisoned in a Gestapo jail in Suwałki.

[18] Several Polish priests were arrested and deported to the Sachsenhausen, Soldau and Dachau concentration camps, where most of them died.

A notable influx of Poles resettled from the Polish areas annexed by the Soviet Union allowed for fast reconstruction of the town.

[23] Currently, Sejny is a notable centre of trade, production and tourism, with thousands of tourists visiting the town every year.

Mayor Arkadiusz Nowalski is fighting to rescue the town, which lacks sufficient collateral for necessary investment or to seek European development funding.