The plant extract that is a natural source of salicylic acid used in skincare is willow bark.
Willow bark contains salicin and related salicylates. When processed by the body, salicin can ultimately produce salicylic acid, the same keratolytic compound used in many acne and exfoliating products. Salicylic acid helps loosen dead skin cells and clear blocked pores.
The connection is ancient: willow preparations were used for pain and fever long before modern chemistry identified their active compounds. In 1828, German chemist Johann Andreas Buchner isolated and named salicin from white willow; later researchers clarified its relationship to salicylic acid.
A common mix-up is assuming willow bark extract and purified salicylic acid are identical. They are not: botanical extracts contain a mixture of compounds, while skincare products usually use measured, purified salicylic acid. Meadowsweet is another plant associated with natural salicylates, but willow bark is the standard answer to this question.