Rensselaer Polytechnic Institute (/rɛnsəˈlɪər/), or RPI, is a private research university and space-grant institution located in Troy, New York, with two additional campuses in Hartford and Groton, Connecticut.

The institute was established in 1824 by Stephen van Rensselaer and Amos Eaton for the "application of science to the common purposes of life" and is described as the oldest technological university in the English-speaking world. Numerous American colleges or departments of applied sciences were modeled after Rensselaer. Built on a hillside, RPI's 265-acre (107 ha) campus overlooks the city of Troy and the Hudson River and is a blend of traditional and modern architecture. The institute operates an on‑campus business incubator and the 1,250-acre (510 ha) Rensselaer Technology Park.