Baron Jöns Jacob Berzelius August 20, 1779 – August 7, 1848), known throughout his life as Jacob Berzelius, is a Swedish chemist.

Berzelius is considered, along with Robert Boyle, John Dalton, and Antoine Lavoisier, to be one of the founders of modern chemistry.

Berzelius begins his career as a physician but his researches in physical chemistry are of lasting significance in the development of the subject.

He is especially noted for his determination of atomic weights; his experiments lead to a more complete depiction of the principles of stoichiometry, or the field of chemical combining proportions.

In 1803 Berzelius demonstrates the power of an electrochemical cell to decompose chemicals into pairs of electrically opposite constituents.

Berzelius's work with atomic weights and his theory of electrochemical dualism leads to his development of a modern system of chemical formula notation that can portray the composition of any compound both qualitatively (by showing its electrochemically opposing ingredients) and quantitatively (by showing the proportions in which the ingredients are united).

His system abbreviates the Latin names of the elements with one or two letters and applies subscripts to designate the number of atoms of each element present in both the acidic and basic ingredients.

Berzelius himself discovers and isolates several new elements, including cerium (1803) and thorium (1828).

Berzelius's interest in mineralogy also fosters his analysis and preparation of new compounds of these and other elements.

The mineral berzelianite is discovered in 1850 and named after him.

He is a strict empiricist and insists that any new theory be consistent with the sum of chemical knowledge.

He develops classical analytical techniques, and investigates isomerism and catalysis, phenomena that owe their names to him.

He becomes a member of the Royal Swedish Academy of Sciences in 1808 and serves from 1818 as its principal functionary, the perpetual secretary.

He is known in Sweden as "the Father of Swedish Chemistry".

Berzelius Day is celebrated on August 20 in his honor.