Western aster is an erect perennial wildflower arising 20-100 cm from a creeping rhizome. The herbage is generally smooth or without hairs. The lower stem leaves are oblanceolate in shape and petiolate, and they tend to persist into flowering. They may have entire or serrate margins. The middle and upper leaves are narrower with entire margins and measure from 3-15 cm long and 3-15 mm wide. The leaves tend to be 7-20 times longer than they are wide.
The inflorescence is a corymb or cymose panicle of one to many flower heads about 2.5 cm wide. The involucre is 5-7 mm high with obtuse to acutely tipped bracts. The bracts are green and occasional purple margined. Individual bracts are narrowly linear or linear-oblong with short hairs along the margins. The 20-50 rays are about 6-15 mm long and blue, violet or purple in color.
Variety occidentalis is shorter and less branched than variety intermedius . It rarely exceeds 50 cm in height and has 1-10 flower heads in a sparsely leafy inflorescence.
Western aster may be found in mountain meadows. Variety occidentalis may be found at higher elevations than variety intermedius.
Aster occidentalis is found from British Columbia south through Washington and Oregon to California, and eastward to Idaho and Colorado.
Variety occidentalis may be found throughout the range of the species with the exception of the eastern portion of its range where variety intermedius becomes prominent.
In the Columbia River gorge, variety occidentalis may be found at an elevation of 4000 between Rainy and North Lakes.