Isaiah David Hart, Sr (November 6, 1792 - September 4, 1861) was an American plantation owner, slave master, and the founder of Jacksonville, Florida. Originally from Georgia, he began setting out the plans for the town in 1822 after moving to the area known as the Cow Ford on the St. Johns River. Hart was a wealthy planter who took arms against Spain in the 1812 Patriot Rebellion and nine years later founded Jacksonville. He had served as postmaster, court clerk, commissioner of pilotage, judge of elections, militia major during the Seminole War and a Whig member of the Florida Territorial Senate. Described as "an eccentric character," Hart owned downtown real estate, a two-story boarding house, a plantation and forty-eight slaves. The Isaiah D. Hart Bridge over the St. Johns River in Jacksonville is named after him.
His son, Ossian B. Hart, became the tenth governor of Florida in 1873.