 The entirety of modern Route 343 was originally the Dover branch of the Dutchess Turnpike. The turnpike, which was in operation from the early to the mid-19th century, was a major transportation route at the time, connecting several local communities to Litchfield County, Connecticut, and the city of Poughkeepsie. NY 343 was designated in 1930, connecting the hamlet of Amenia to the state line, but was relocated a few years later onto the portion of New York State Route 200 from South Millbrook to the hamlet of Dover Plains. The NY 200 designation was transferred to NY 343's original alignment. NY 343 absorbed NY 200 in the early 1940s, creating an overlap with NY 22 between Dover Plains and Amenia. The Connecticut portion of the highway was originally designated as part of Route 4; however, it was renumbered to Route 343 in 1932.
