James Rumsey (1743 – December 21, 1792) was an American mechanical engineer chiefly known for exhibiting a boat propelled by machinery in 1787 on the Potomac River at Shepherdstown in present-day West Virginia before a crowd of local notables, including Horatio Gates. A pump driven by steam power ejected a stream of water from the stern of the boat and thereby propelled the boat forward.

Little is known about Rumsey until he was living in Bath, Virginia, (now Berkeley Springs, West Virginia) in 1782. He likely had moved to the area with his family some years before the American Revolution, from Cecil County, Maryland, where he had helped to run the family water mill at Bohemia Manor. His cousin was Benjamin Rumsey, a notable Maryland jurist and statesman, who also grew up at Bohemia Manor. In Bath, he built houses, became a partner in a mercantile business, and helped to run a boarding house and tavern called the "Sign of the Liberty Pole and Flag."

In September 1784, when George Washington was staying at Rumsey's inn, he contracted with Rumsey to build a house and stable on property he owned at Bath. During this visit, Rumsey showed Washington a working model of a mechanical boat which he had designed. It had a bow-mounted paddlewheel that worked poles to pull the boat upstream. Washington had been making plans for making the Potomac river navigable since before the Revolution, and a company was soon to be formed for the purpose. Rumsey's pole-boat, which promised to be able to ascend the river's chutes and swift currents, must have seemed a godsend to Washington, who wrote a certificate of commendation for Rumsey and likely let him know of the river project. Armed with the certificate, Rumsey obtained a patent from the Virginia legislature for "the use of mechanical boats of his model" and also gained an investor James Mcmechen.

In July 1785, he was recommended by Washington and appointed the superintendent of the newly formed Patowmack Company to oversee the clearing of rocks at what is now Harper's Ferry. Rumsey would thus not only be able to build a boat to ascend the river, but alter the river to enable his boat. For a year, Rumsey oversaw work on the Potomac River site, while his assistant and brother-in-law Joseph Barnes did much of the building of the boat around Shepherdstown.

Rumsey had quickly concluded that the pole-boat design was too limited and decided to incorporate steam propulsion into his design. While making his boat much more useful, it made it far more complex and expensive to build. It soon became obvious that the Patowmack Company had a much greater task ahead than any of its members had foreseen. It was hindered by the lack of an overall supervising engineer; overseers were having to improvise as best they could. The work required much manual labor and difficult blasting, and Rumsey found himself directing a large and restive gang of about a hundred workmen, including leased slaves and bondsmen, encamped in a remote area, without adequate supplies.

After a year Rumsey said he would resign if not given an increase in pay. His resignation was accepted and his assistant, Richardson Stewart, was given his job. Other aspects of the matter are open to debate; Stewart may or may not have worked against Rumsey to gain his job; Rumsey thought he had, the Company (and Washington) thought Rumsey's allegations unfounded. Still, according to Company minutes, Stewart was fired soon afterward, for "sundry charges of a serious nature".

Work on a hull had begun in 1785 in Bath by Joseph Barnes. The boat was brought that fall to Shepherdstown. Valve castings, cylinders, and other pieces which had been made in Baltimore and Frederick were installed that December, and the boat was taken downriver to Shenandoah Falls for a test. However, bad weather postponed testing until the following spring. When Rumsey finally tested the boat, it proved very unsatisfactory. The pole-boat mechanism caused the boat to yaw in the current, which disabled the paddlewheel and stopped the boat. In the steam pump, the engine consumed too much steam; the boiler was inadequate.

At some point in 1786, work on the pole-boat mechanism was abandoned. For a better boiler, he tried a coil of forged iron pipe, which proved to be both much more efficient and much smaller and lighter. With a functioning steam engine, another problem arose. The single-cylinder pump would draw several gallons of water from beneath the boat and send it down a copper pipe to the stern. Because gallons of water were being drawn into the pump at the same time as water was still flowing from it to the stern, the pump was working against itself; several strong strokes and it bound up. This was resolved by replacing the copper pipe with a square wooden trunk with flapper valves in the bottom to allow water in from the river, to relieve the negative pressure at the pump.