Rhododendron tomentosum is a shrub found in coniferous forests, forest margins, marshes and wet meadows; elevations of occurrence are from 400 to 1400 metres. The species has a very broad distribution that includes northern Europe, North America and Asia. Northern North America occurrences include Greenland, Canada and Alaska. Asian occurrences are in China, Korea and Japan. This shrub thrives in peaty soils, especially in moss and lichen tundra. A specific example ecoregion of occurrence is in the South Hudson Bay taiga, particularly on the floor of Black spruce dominant forests, where there is considerable local topographic depressions that create localised wetlands.
Known by the common name Marsh Labrador tea, this plant exhibits an erect or creeping growtn form and can achieve a height of one half metre. The branches are thin with young branches densely covered with a rusty-coloured wool. Terminal ovoid buds are conspicuous, with scales densely rufous-woolly. Leaf blades are linear, linear-lanceolate, or narrowly oblong. Inflorescences manifest in a multi-flowered form.