Harmonia hallii is an annual herb growing up to about 18 centimeters in maximum height, its stiff, leafy stem branching or not.

It is coated in rough hairs, dark-colored stalked resin glands, and bristles.

The glandular inflorescence bears one or more flower heads with yellow ray florets up to half a centimeter long and sometimes tinged with red at the bases.

Harmonia hallii is endemic to California, where it is known only from a small section of the North Coast Ranges.

[1] It grows in chaparral on serpentine soils, and its habitat is limited and being lost to activities such as mining.