valley district, east-central Poland. Located in the eastern part of the central lowlands, it is directly south of the Masurian Lakeland and west of the Podlasian Lowland along the border with Belarus.

The distinctive feature of this sinuous valley is its marshy floodplain, which is bordered by dry fertile river terraces. The valley widens in places, especially in the ancient lake basin of Warsaw, where water was once dammed up in front of a glacial ice sheet. The Vistula River follows a serpentine course through the broad valley; in places the river, bordered by marshes, is subject to flooding. Elsewhere in the lowland, sand deposits have piled up and made the soil useless for cultivation.

The distinctive settlement feature of the Mazovian Lowland is the city of Warsaw, the national capital of Poland. The city is located above the flood level, mainly on the left bank of the Vistula. Płock and Siedlce are other important urban settlements in the region.