Matthias Eugen Oscar Liebreich (14 February 1839 – 2 July 1908) was a German pharmacologist.

Beginning in 1867, he worked as an assistant in the chemistry department of the pathological institute under Rudolf Virchow.

Liebreich introduced the method of phaneroscopic illumination for the study of lupus; showed the value of cantharidin in tuberculosis, of mercuric formamide and of lanolin in syphilis, of butylchloral hydrate and of ethylene chloride[note 1] as anesthetics.

In 1865, he gave the name "protagon" to a proximate principle discovered in the brain and in blood corpuscles.

He is well known for his investigations pertaining to the sedative and hypnotic properties of chloral hydrate (1869), and was an important factor in the drugs' popularity during the latter half of the 19th century.