Macierowe Bagno, Peatland in Wesoła, Poland.

Macierowe Bagno is a peatland covering approximately 5 hectares and characterized by water-saturated soil and specialized plant species. The area supports vegetation uniquely adapted to wet conditions, creating a distinct ecosystem within the region.

Peat was dug from parts of the bog in various periods, while other sections were farmed. A road was built on a causeway and two ponds were created, and a holiday-cottage settlement grew up beside the wetland. The bog lies within the Mazowieckie Landscape Park.

Locals call the peatland "Macierówka". It lies by Torfowa Street in Stara Miłosna, about 100 m north of the Zielony Ług peat bog with its small pond; some sources treat Macierowe Bagno as the northern part of Zielony Ług. The surrounding area was partly dug for peat, farmed, crossed by a road on a causeway, and built over with holiday cottages.

Macierowe Bagno lies near Torfowa Street in Stara Miłosna, about 100 m north of the Zielony Ług peat bog and its small pond, with the Czarny Staw reservoir further southwest. Water levels swing by 29–56 cm with rainfall, and after the heavy rains of 2010 the bog looked like a lake in summer 2012. Parts were drained, dug for peat and farmed, and a road was built on a causeway, so wet ground is likely.

The peatland is one of the few places in the Mazovian Landscape Park where the intermediate sundew, Drosera intermedia, was recorded. After exceptionally heavy rainfall in 2010 raised water levels, the species probably died out there; by summer 2012 the bog looked like a lake. Researchers found water levels swinging 29–56 cm during 2008–2009 surveys.