Red alder is the largest species of alder in North America and one of the largest in the world, reaching heights of 20 to 30 m (66 to 98 ft). The official tallest red alder (1979) stands 32 m (105 ft) tall in Clatsop County, Oregon (US). The name derives from the bright rusty red color that develops in bruised or scraped bark. The bark is mottled, ashy-gray and smooth, often colonized by white lichen and moss. The leaves are ovate, 7 to 15 centimetres (2.8 to 5.9 in) long, with bluntly serrated edges and a distinct point at the end; the leaf margin is revolute, the very edge being curled under, a diagnostic character which distinguishes it from all other alders. The leaves turn yellow in the autumn before falling. The male flowers are dangling reddish catkins 10 to 15 cm (3.9 to 5.9 in) long in early spring. Female flowers occur in clusters of (3) 4–6 (8). Female catkins are erect during anthesis, but otherwise pendant. They develop into small, woody, superficially cone-like oval dry fruit 2 to 3 cm (0.79 to 1.18 in) long. The seeds develop between the woody bracts of the 'cones' and are shed in late autumn and winter. Red alder seeds have a membranous winged margin that allows long-distance dispersal.
Alnus rubra grows from southeast Alaska south to central coastal California, nearly always within about 200 km (120 mi) of the Pacific coast, except for an extension 600 km (370 mi) inland across Washington and Oregon into northernmost MontanaIn southern Alaska, western British Columbia and the northwestern Coast Ranges of the United States, red alder grows on cool and moist slopes; inland and at the southern end of its range (California) it grows mostly along the margins of watercourses and wetlands.Red alder is associated with coast Douglas-fir (Pseudotsuga menziesii subsp. menziesii), western hemlock (Tsuga heterophylla), grand fir (Abies grandis), western redcedar (Thuja plicata), and Sitka spruce (Picea sitchensis) forests.
Along streambanks it is commonly associated with willows (Salix spp.), red osier dogwood (Cornus stolonifera), Oregon ash (Fraxinus latifolia), and bigleaf maple (Acer macrophyllum).To the southeast of its range it is replaced by white alder (Alnus rhombifolia), which is a tree of similar stature, but which differs in the leaf margins not being rolled under, lack of distinct lobes, and lack of membranous wings on seed margins. In the high mountains it is replaced by the smaller and more shrub-like Sitka alder (Alnus viridis subsp. sinuata), and east of the Cascade Mountains by thinleaf alder (Alnus incana subsp. tenuifolia).