Axel Hugo Theodor Theorell (6 July 1903 – 15 August 1982) was a Swedish scientist and Nobel Prize laureate in medicine.

He was born in Linköping as the son of Thure Theorell and his wife Armida Bill. Theorell went to Secondary School at Katedralskolan in Linköping and passed his examination there on 23 May 1921. In September, he began to study medicine at the Karolinska Institute and in 1924 he graduated as a Bachelor of Medicine. He then spent three months studying bacteriology at the Pasteur Institute in Paris under Professor Albert Calmette. In 1930 he obtained his M.D. degree with a theory on the lipids of the blood plasma, and was appointed professor in physiological chemistry at the Karolinska Institute.

Theorell, who dedicated his entire career to enzyme research, received the Nobel Prize in Physiology or Medicine in 1955 for discovering the oxidation enzyme and its effects. He was a former head of research of the Nobel Institute, the first researcher related to the Institute to be awarded a Nobel Prize.

His work had led to pioneering progress on ADH enzymes, which break down alcohol in the kidney. His work won praise in Sweden as well as around the world. And he received honorary degrees at universities in France, Belgium, Brazil and the United States.

Theorell died in Stockholm and is buried in Norra begravningsplatsen (The Northern Cemetery) alongside his wife, Elin Margit Elisabeth (née Alenius) Theorell, a distinguished pianist and harpsichordist.