James Bowdoin II (/ˈboʊdɪn/ BOH-din; August 7, 1726 – November 6, 1790) was an American politician from Boston, Massachusetts who was active during the American Revolution and the following decade. He initially gained fame and influence as a wealthy merchant. He served in both branches of the Massachusetts General Court from the 1750s to the 1770s. Although he was initially supportive of the royal governors, he opposed British colonial policy and eventually became an influential advocate of independence. He authored a highly political report on the 1770 Boston Massacre that has been described by historian Francis Walett as one of the most influential pieces of writing that shaped public opinion in the colonies.

From 1775 to 1777, he served as president of the Massachusetts Provincial Congress's executive council, the de facto head of the Massachusetts government. He was elected president of the constitutional convention that drafted the state's constitution in 1779, and ran unsuccessfully for governor in 1780, losing to John Hancock. In 1785, following Hancock's resignation, he was elected governor. Due to the large debts of Massachusetts, incurred from the Revolutionary War, Bowdoin ran on a platform of fiscal responsibility. During his two years in office, the combination of poor economic conditions and his harsh fiscal policy laid down by his government led to the uprising known as Shays' Rebellion. Bowdoin personally funded militia forces that were instrumental in putting down the uprising. His high-handed treatment of the rebels may have contributed to his loss of the 1787 election, in which the populist Hancock was returned to office.

In addition to his political activities, Bowdoin was active in scientific pursuits, collaborating with Benjamin Franklin in his pioneering research on electricity. He was elected a Fellow of the Royal Society of London and was elected a member of the American Philosophical Society in 1787. He was a founder and first president of the American Academy of Arts and Sciences, to whom he bequeathed his library. Bowdoin College in Maine was named in his honor after a bequest by his son James III.

James Bowdoin II was born in Boston to Hannah Portage Bowdoin and James Bowdoin, a wealthy Boston merchant. His grandfather, Pierre Baudouin, was a Huguenot refugee from France. Pierre took his family first to Ireland, then to eastern Massachusetts (present-day Maine), before finally settling in Boston in 1690. James Bowdoin I had a modest inheritance from his parents, but greatly expanded his father's merchant business and land holdings to become one of the wealthiest men in the province. Young James attended the South Grammar School (now Boston Latin School), then graduated from Harvard College in 1745. When his father died in 1747, he inherited a considerable fortune. He married Elizabeth Erving, sister of his Harvard roommate, in 1748. They had two children. That same year, he received his master's degree from Harvard.

Bowdoin may have met Benjamin Franklin as early as 1743, and the two became frequent collaborators and correspondents on scientific subjects. During his Harvard years, he was educated in the sciences by John Winthrop, and developed an interest in electricity and astronomy. In 1750, Bowdoin traveled to Philadelphia to meet with Franklin. Bowdoin was interested in Franklin's experiments on electricity, and Franklin solicited his advice on papers he prepared for submission to the Royal Society. Through the offices of Franklin, some of Bowdoin's letters were read to the Society. Bowdoin was instrumental in gaining support in the provincial assembly for an expedition to Newfoundland to observe the 1761 transit of Venus across the sun, and in the same year published a treatise suggesting improvements to the telescope. In 1785 he published a series of memoirs arguing against Isaac Newton's theory that light was transmitted by "corpuscles", citing both natural observations and Scripture.

Bowdoin maintained a lifelong interest in the sciences. In 1780 he was one of the founders of the American Academy of Arts and Sciences. He served as its first president until his death and left the society his library. Bowdoin published not only scientific papers, but poetry in both English and Latin. He was awarded an honorary doctorate by the University of Edinburgh and made a fellow of Harvard. His 1788 election to the Royal Society of London was the first such honor bestowed on an American after independence.

Bowdoin also had extensive business interests. Although he was often characterized as a merchant, and he engaged in the Atlantic trade, his principal interest was in land. His inheritance included major tracts of land, most of which he kept, in present-day Maine as well as in the agriculturally rich Elizabeth Islands off the state's south coast. Bowdoin expanded his holdings, eventually acquiring property in all of the New England states except Rhode Island. He was one of the managing proprietors of a large territory on the Kennebec River, where he was frequently involved in legal proceedings with squatters on the land, and with competing land interests. The dealings with squatters in particular left Bowdoin with a dislike of the lower classes in Massachusetts society, something that affected his politics. His inheritance also included an ironworks in Attleboro (now Bridgewater) that he sold in 1770, apparently because it was too time-consuming to manage. Despite the upheavals of the Revolution, Bowdoin was careful to always manage his financial affairs. He supported the cause of independence financially, but he did so without damaging his own business interests, unlike John Hancock, whose business suffered from neglect.

In later years he served as the first president of the Massachusetts Bank in 1784 and was also the first president of the Massachusetts Humane Society (an organization initially devoted to rescuing survivors from shipwrecks and other water-based disasters).