The former Dominican complex is the big architectural presence in Sejny. The church was begun in the early 17th century, after the Dominicans were brought from Vilnius, and it still gives the small town a surprisingly formal centre.
Go inside if it is open, then walk around the old monastery buildings and the square. The complex helps explain why Sejny was more than a roadside settlement: religion, trade, and border politics all passed through this small hill above the Marycha River.