Stein Bjornar Jacobsen (born 1950)^[1] is a Norwegian-American geochemist who works within cosmochemistry.

Hailing from Drammen, he finished a cand.mag. degree at the University of Oslo before studying geology in California with a Rotary grant.^[2] Jacobsen became a professor of geochemistry at Harvard University.^[3]

He was an inducted into the Norwegian Academy of Science and Letters in 1994.^[1] In 2009 he was inducted into the American Academy of Arts and Sciences, mainly for using "the distribution of long-lived and extinct radioisotopes to date the formation of the earth's core and to define the effects of core separation on the early history of the core-mantle-crust system".^[4]