Helianthus decapetalus, known by the common names thinleaf sunflower, thin-leaved sunflower, and woodland sunflower, is a perennial forb in the family Asteraceae. It is native to the Eastern and Central United States and Canada, from New Brunswick west to Iowa, Wisconsin, and Ontario, south as far as Georgia and Louisiana. It produces yellow composite flowers in late summer or early fall.

The name decapetalus means "with ten petals", referring to the 8-12 ray florets on the flower heads, resembling petals.

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