Alexander Robertus Todd, Baron Todd, OM, PRS, FRSE (2 October 1907–10 January 1997) was a Scottish biochemist whose research on the structure and synthesis of nucleotides, nucleosides, and nucleotide coenzymes gained him the 1957 Nobel Prize for Chemistry.

Todd became the Sir Samuel Hall Professor of Chemistry and Director of the Chemical Laboratories of the University of Manchester in 1938, where he began working on nucleosides, compounds that form the structural units of nucleic acids (DNA and RNA). In 1944 he was appointed to the 1702 Chair of Chemistry in the University of Cambridge, which he held until his retirement in 1971, after a heart attack. In 1949 he synthesized adenosine triphosphate (ATP) and flavin adenine dinucleotide (FAD).
In 1955 he elucidated the structure of vitamin B12, later working on the structure and synthesis of vitamin B1 and vitamin E, the anthocyanins (the pigments of flowers and fruits) from insects (aphids, beetles) and studied alkaloids found in hashish and marijuana. He served as chairman of the British government's advisory committee on scientific policy from 1952 to 1964.
He was elected a Fellow of Christ's College, Cambridge in 1944 and was Master from 1963 to 1978. He became Chancellor of the University of Strathclyde in 1975, and a visiting professor at Hatfield Polytechnic (1978–1986). Among his many honours, including over 40 honorary degrees, he was elected as a Fellow of the Royal Society in 1942, was President of the Royal Society from 1975 to 1980 and became a member of the Royal Order of Merit in 1977.
Lord Todd was married to Alison Sarah, daughter of Nobel Prize winner Sir Henry Dale, and had a son, Alexander Henry, and two daughters, Helen Jean and Hilary Alison.