Cassinia longifolia, commonly known as shiny cassinia, is a species of flowering plant in the family Asteraceae and is endemic to eastern Australia. It is an erect, aromatic shrub with sticky, hairy foliage, linear or oblong to narrow lance-shaped leaves, and heads of creamy-white flowers arranged in a dense corymb.
Cassinia longifolia is an erect, aromatic shrub that typically grows to a height of , its foliage covered with short, glandular hairs and sticky. The leaves are linear or oblong to narrow lance-shaped, long and wide on a petiole long. The upper surface of the leaves is , the edges curve downwards and the lower surface is covered with fine hairs. The flower heads are long and wide, each with five or six creamy-white florets surrounded by three to five overlapping rows of egg-shaped involucral bracts. The heads are arranged in a dense corymb up to in diameter. Flowering occurs in summer and autumn and the achenes are about long with a pappus long.
Cassinia longifolia was first formally described in 1818 by Robert Brown in the Transactions of the Linnean Society of London. The specific epithet (longifolia) means "long-leaved".
Shiny cassinia grows in forest and disturbed places, especially after fire, and often on ridges. It occurs in south-eastern New South Wales, the Australian Capital Territory and is widespread and common in eastern Victoria.