thumb|right|300px|Bridge over the Walloomsac River, Hoosick Falls, from a 1907 postcard
The Walloomsac River () from the Native American name, Wal-loom-sac is a <!--Length given for river as shown on topo maps, from confluence of South Stream and Jewett Brook--> tributary of the Hoosic River in the northeastern United States. It rises in southwestern Vermont, in the Green Mountains east of the town of Bennington in Woodford Hollow at the confluence of Bolles Brook and City Stream where it is labeled Walloomsac Brook on maps but is locally known as "The Roaring Branch". The river then flows west toward Bennington and passes the downtown area to the north. For many years this section was intermittent due to the water having been diverted to power mills in town (ca. 1810). This divergence gave the name Walloomsac to a portion of the river flowing through town on the present course of South Stream.