Wąsosz [ˈvɔ̃sɔʂ] is a village in Grajewo County, Podlaskie Voivodeship, in north-eastern Poland. It is the seat of the gmina (administrative district) called Gmina Wąsosz. It lies approximately 17 kilometres (11 mi) south-west of Grajewo and 73 km (45 mi) north-west of the regional capital Białystok.

It is situated on the Wissa River, a tributary of the Biebrza.

In 1435 Wąsosz was purchased by Duke Władysław I of Płock. It received Chełmno municipal rights from Duke Władysław I on 13 May 1436, and lost them in 1870 under the Russian rule following the Partitions of Poland. In 1605 local burgher Andrzej Rogala founded a Carmelite monastery. The already well-developed town was destroyed in the Swedish Deluge of 1655-1656 and then rebuilt. The town hall was erected in 1789. Almost all of the streets were paved at around the same time. The town lost its importance due to the development of the nearby town of Szczuczyn and the bypassing of Wąsosz by a new route connecting Warsaw and Kowno. The Carmelite monastery was closed in 1864 by the Tsarist authorities in reprisal for help offered by monks to victims of the January Uprising. They were sent to Katorga chained by the neck.

Following the joint German-Soviet invasion of Poland, which started World War II in September 1939, it was first occupied by the Soviet Union until 1941, and then by Germany until 1944. On the night between 4 and 5 July 1941, during the Nazi invasion into Eastern Poland and the USSR, a small group of people murdered several dozens of the Jewish inhabitants of Wąsosz, in what is called the Wąsosz pogrom.

One of the most impressive points of interest in Wąsosz is its late Gothic church from 1508, with three altars adorned with religious paintings. It is at the Old Market Square. There is also a Baroque Church of the Nativity of the Virgin Mary from the early 17th century.