Siennica [ɕɛnˈnit͡sa] is a town in Mińsk County, Masovian Voivodeship, in east-central Poland.

In 1526 Duchess Anna of Masovia founded the town of Janów at the site of Siennica.

A chapel was built next to the church, where members of the local noble Siennicki family were buried.

In 1864 the Russian imperial authorities liquidated the monastery as part of their anti-Polish repressions after the unsuccessful Polish January Uprising; nevertheless, two years later a theological college was founded there by the Catholics.

Following Operation Barbarossa they deported about 700 Jewish residents of town to extermination camps.