The River Terrig () is a small river in north-east Wales.
The river rises at Llyn Cyfynwy near Graianrhyd village in the community of Llanarmon-yn-Ial, Denbighshire, about three miles from the source of the River Alyn.Cambridge County Geographies: Flintshire, Cambridge University Press, p.25 It then flows northwards and eastwards, forming the boundary between the old parishes of Nercwys and Treuddyn. At Nant-y-Mynydd it is joined by several small springs from Mynydd Ddu, and finally itself joins the River Alyn at Pontblyddyn, Flintshire.
The Terrig is a habitat for brown trout. Its name is derived from its rapid flow after times of heavy rain; Thomas Pennant, in his Tours in Wales, described it as "the Terrig, or the violent, [...] often of a tremendous swell and fury".Pennant, T. Tours in Wales, Volume 2, Wilkie and Robinson, 1810, p.44
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