Muskeg (Ojibwe: mashkiig; Cree: maskīk; French: fondrière de mousse, lit. moss bog) is a peat-forming ecosystem found in several northern climates, most commonly in Arctic and boreal areas. Muskeg is approximately synonymous with bog or peatland, and is a standard term in Western Canada and Alaska. The term became common in these areas because it is of Cree origin; maskek (ᒪᐢᑫᐠ) meaning "low-lying marsh".

Muskeg consists of non-living organic material in various states of decomposition (as peat), ranging from fairly intact sphagnum moss, to sedge peat, to highly decomposed humus. Pieces of wood can make up five to fifteen percent of the peat soil. The water table tends to be near the surface. The sphagnum moss forming it can hold fifteen to thirty times its own weight in water, which allows the spongy wet muskeg to also form on sloping ground.

Muskeg patches are ideal habitats for beavers, pitcher plants, agaric mushrooms and a variety of other organisms.