John Harris (1754 – 27 April 1838) was a military surgeon and magistrate who became one of the earliest major landowners in the Colony of New South Wales, Australia.
Harris was born in Moneymore, County Londonderry, Ireland, and studied medicine at the University of Edinburgh in Scotland. Prior to his arrival in New South Wales in June 1790, he had served in the Royal Navy for 10 years.
Harris was granted 100 acres (0.4 km2) of land in Parramatta in 1793. This grant was cancelled a year later and he was granted 110 acres (0.4 km2) in Harris Park instead.
In 1803 he was brought before a court composed of officers of the New South Wales Corps, charged with committing an offence on the "19th ultimo" (the 19th of the previous month). As the offence had been committed on the "19th instant" (the 19th of the current month), he had to be acquitted. In 1806, he was granted all of the land on the Drummoyne Peninsula. He had various other landholdings including some in Ultimo, which he named after his famous legal case. His total landholdings were listed at 5,114 acres (21 km2) in 1826.
Despite having no formal legal training, Harris was appointed a magistrate and played an important role in the various political manoeuvrings that took in the colony during its first two decades. Among his colleagues during this pivotal early period was his fellow Irish-born surgeon and magistrate, Thomas Jamison. In 1817, he was one of the founding directors of the Bank of New South Wales.