The Ompompanoosuc River /ˈɒmpɒmpənuːsək/ is a river, about 25 mi (40 km) long, in eastern Vermont in the United States. It is a tributary of the Connecticut River, which flows to Long Island Sound. According to the Geographic Names Information System, the river has also been known by the names "Om-pom-pa-noos-uc" and "Pompanoosuc." The Ompompanoosuc River rises in eastern Orange County in the town of Vershire, and flows generally southwardly through the towns of West Fairlee and Thetford into northeastern Windsor County, where it joins the Connecticut River in the town of Norwich.