Erik Acharius (10 October 1757 – 14 August 1819) was a Swedish botanist who pioneered the taxonomy of lichens and is known as the "father of lichenology".

Acharius was born in Gävle, matriculated at Uppsala University in 1773 and was one of the last of the students of Linnaeus. He later worked for the Royal Academy of Sciences in Stockholm and completed his medical studies at Lund University in 1782. He was appointed town medical officer in Vadstena in 1785, district medical officer in Östergötland County in 1789, director of the new Vadstena Hospital (which he had initiated) in 1795, and titular professor in 1803.

Acharius belonged to the younger generations of Swedish botanists who continued what Linnaeus had left undone. Acharius began the taxonomic classification of the Lichenes and published several works in this field: Lichenographiae Suecia prodromus (1798), Methodus lichenum (1803), Lichenographia universalis (1810), Synopsis methodica lichenum (1814) and many smaller papers in periodicals.

He was a member of the Royal Physiographic Society in Lund (1795), the Royal Swedish Academy of Sciences (1796), the Linnean Society of London (1801) and the Royal Society of Sciences in Uppsala (1810).

The genus Acharia, several plants, such as Rosa acharii and Conferva acharii and one insect, Tortrix achariana have been named after Acharius; likewise, the Acharius Medal is awarded for lifetime achievement in lichenology.

The collections of Acharius are distributed over several museums: the Finnish Museum of Natural History in Helsinki, the Botanical Museum in Uppsala, the Swedish Museum of Natural History and the Botanical Museum in Lund. His papers are in the Library of Uppsala University.