The Hardenberg/Rhinelander Historic District is a small historic district in the Carnegie Hill neighborhood of the Upper East Side of Manhattan, New York City. It was created by the New York City Landmarks Preservation Commission on May 5, 1998, and consists of six brick, brownstone and terra cotta Northern Renaissance Revival rowhouses along Lexington Avenue between East 89th and 90th Streets, and one apartment building, referred to as "French Flats" at the time, on East 89th Street. All the buildings were constructed in 1888–1889.