Solidago gigantea develops clones of stems often 1 meter tall or more (50-200+ cm) with conspicuous, roughly triangular inflorescences of many small, bright yellow flowers in heads. The flower heads are secund, meaning that the pedicels by which they are attached tend to bend around the branch so that all or most heads appear to arise from the top of the branch. The leaves are simple and alternate, 6-18 cm long and about 1/4 as wide and tapering to both ends. The leaf margins are sharply toothed, at least in the distal half, and the leaves generally have two larger lateral veins that, together with the midrib, give them a conspicuous 3-nerved appearance.

Solidago gigantea is similar in general form to S. canadensis and S. altissima and two or more of these species may occur in close proximity to one another. S. gigantea can be distinguished from the other two species by its glabrous and (often) glaucus stem below the inflorescence (the upper part of the stem, as it becomes the axis of the inflorescence, is usually pubescent, however). The stem in the other two species is pubescent below the inflorescence (though not necessarily all the way to the base).

Beginners are advised to read the identification of goldenrod species.