Samuel Hartlib or Hartlieb (c. 1600 – 10 March 1662) was a polymath of German-Polish origin, who settled, married and died in England. He was the son of Georg Hartlieb, a German merchant in Elbing (Elbląg), whose family had migrated to Poland at the beginning of the 16th century, and Elizabeth Langthon, a daughter of a rich English merchant in Danzig (Gdańsk). Hartlib was a noted promoter and writer in fields that included science, medicine, agriculture, politics and education. He was a contemporary of Robert Boyle, whom he knew well, and a neighbour of Samuel Pepys in Axe Yard, London, in the early 1660s. He studied briefly at the University of Cambridge upon arriving in England.

Hartlib was born in Elbląg (Elbing), Royal Prussia, Polish-Lithuanian Commonwealth. His mother was the daughter of a rich English merchant in Danzig (Gdańsk). The family of his father, Georg Hartlieb, migrated from Germany to Poland at the beginning of the 16th century. His father was Protestant and settled in Poznań. Due to increasing persecution of Protestants, Georg Hartlieb left Poznań in 1579, relocating first to Danzig and shortly thereafter to Elbing, where he established himself as a successful businessman. Samuel Hartlib studied at the Gymnasium in Brieg (Brzeg) and at the Albertina. He went on to Herborn Academy, where he studied under Johannes Heinrich Alsted and Johannes Bisterfeld. Although briefly at the University of Cambridge, supported by John Preston, he does not seem to have formally studied there.

Hartlib met the Scottish preacher John Dury in 1628. In the same year, Hartlib relocated to England, faced with the prospect of being caught in a war zone, as Imperial armies moved into the western parts of Poland and the chance of intervention by Sweden grew. He first unsuccessfully set up a school in Chichester, in line with his theories of education, and in 1630 moved permanently to London, living in Duke's Place, Holborn. An early patron was John Williams, the Bishop of Lincoln, who was leading the clerical opposition to Archbishop William Laud. Another supporter was John Pym; Pym would use Hartlib later, as a go-between with Dutch Calvinists in London, in an effort to dig up evidence against Laud. Hugh Trevor-Roper argues in his essay Three Foreigners (referring to Hartlib, Dury and the absent Comenius) that Hartlib and the others were the "philosophers" of the "country party" or anti-court grouping of the 1630s and early 1640s, united in their support for these outside voices if agreeing on little else.

During the Civil War, Hartlib occupied himself with the peaceful study of agriculture, publishing various works of his own and printing at his own expense several treatises by others on the subject. He planned a school for the sons of gentlemen, to be conducted on new principles, and this probably was the occasion of his friend John Milton's Tractate on Education, addressed to him in 1644, and of William Petty's Two Letters on the same subject, in 1647 and 1648. Another associate in that period was Walter Blith, a noted writer on husbandry.

For his various labours, Hartlib received a pension of £100 from Oliver Cromwell, afterwards increased to £300, as he had spent all his fortune on his experiments. But Hartlib died in poverty: Samuel Pepys in 1660 noted that Hartlib's daughter Nan was penniless. His association with Oliver Cromwell and the Commonwealth resulted in him being sidelined after Charles II's Restoration. He lost his pension, which had already fallen into arrears. Some of his correspondents went so far as to ask for their letters from his archive, fearing they could be compromised by them.

Hartlib is often described as an "intelligencer", and indeed has been called "the Great Intelligencer of Europe". His main aim in life was to further knowledge. He kept in touch with an array of contacts from high philosophers to gentleman farmers. He maintained a voluminous correspondence, lost in 1667, but much recovered since 1945; it is housed in a special Hartlib collection at the University of Sheffield, England.

Hartlib became one of the best-connected intellectual figures of the Commonwealth era. He was responsible for patents, spreading information and fostering learning. He circulated designs for calculators, double-writing instruments, seed machines and siege engines. His letters in German, Latin, English and other languages have been subjected to close modern scholarship.

Hartlib set out with a universalist goal: "to record all human knowledge and to make it universally available for the education of all mankind". His work has been compared to modern internet search engines.