Adrian Georg Iselin (January 17, 1818 – March 28, 1905) was a New York financier who invested in and developed real estate, railroads, and mining operations. For many years during his early business career he was engaged in importing with his brother, William Iselin, being one of the most successful merchants of New York in the middle of the century. After retiring from the importing trade, he established the banking house of Adrian Iselin & Co. He is considered the founder of the Iselin family in the United States.

Iselin was born in Scotland on January 17, 1818, while his parents were making a tour of the British Isles. He was fifth of eleven children born to Isaac Iselin (1783–1841), who was born in Basel, Switzerland, and emigrated to the United States in 1801, and Aimee Jeanne (née Roulet) Iselin (1792–1873), a Swiss-French daughter of John Roulet.

In Switzerland, the Iselin family had been merchants, public officials, and military and professional men since the 14th century. Adrian's father Isaac, who began his career in New York at Gouverneur & Kemble and later at LeRoy, Bayard and McEvers, amassed a large fortune in the importing business which was passed down to him and his siblings.

Iselin was educated abroad in Switzerland. However, he is also shown to have been matriculated at one point at Columbia College with the class of 1837 but did not graduate, according to official documents.

Iselin first joined the New York dry goods company of Messers, Cattenet, Barbey and Co., shortly thereafter going independent and in 1840 forming Moran & Iselin, an imported dry goods firm that expanded to become involved with banking. When Moran retired in 1854, Iselin reorganized and continued the banking arm of the business under A. Iselin and Co. at his office located at 36 Wall Street.

Iselin was one of the directors of the Sioux City and St. Paul Railroad Company in Minnesota. The town of Adrian, Minnesota, which developed after completion of the rail lines, was named after his mother, Mrs. Adrian Iselin. In 1881, Iselin formed the Pittsburgh and State Line Railroad after Rochester and State Line Railroad was put into receivership.

Iselin eventually became the chief investor in the Rochester and Pittsburgh Coal and Iron Company. At the outbreak of the Civil War, his firm was so strong that it helped to finance the United States Government. In 1885, the Rochester and Pittsburgh Coal and Iron Company constructed the world's longest string of coke ovens in Walston, Pennsylvania, with 475 ovens over a length of 2 km (1.25 miles). Their output reached 22,000 tons per month.

At his death, he left the management of his investments and companies to two of his sons, Adrian Jr. and Columbus. Adrian Jr., who joined the firm in 1868, was responsible for the expansion of the Company into Indiana County, Pennsylvania. Ernest Iselin, son of Adrian Jr., was director of the company from 1929 to 1934 and chairman of the board from 1936 to 1954. The firm was in existence until 1936 when it was merged with Dominick & Dominick, an investment and merchant banking firm that exists to this day.