Harris Charles Fahnestock (February 27, 1835 – June 4, 1914) was an American investment banker.

He was a direct descendant of Johann Diedrich Fahnestock, who came to America from Germany in 1726, settling near Ephrata, Pennsylvania.

While in Washington, he began his interest in railroads, becoming treasurer of the Washington & Georgetown Line before moving to New York City in 1866 as a member of Jay Cooke, McCulloch & Co., alongside Cooke and Hugh McCulloch (the former Comptroller of the Currency under Lincoln).

[6] Fahnestock donated $50,000 towards the construction of the west arch or "crossing" of the Cathedral of St. John the Divine in Upper Manhattan.

In addition to serving as a trustee and treasurer of the Metropolitan Museum of Art, and a member and patron of the American Museum of Natural History, he was a benefactor of the Post-Graduate Hospital, to which he donated a significant amount after the death of his wife in 1898, including $100,000 for a Nurses' Training School in 1899.

Fahnestock died on June 4, 1914, at his home, 457 Madison Avenue in Manhattan (which was owned by Random House in 1968 and today is site to Municipal Art Society),[10] after "two weeks' illness from erysipelas and a complication of diseases".