Skarnsund or Skarnsundet is a strait in the Trondheimsfjord in Trøndelag county, Norway. The strait connects the Beitstadfjorden with the outer section of the Trondheimsfjorden. The 5-kilometre (3.1 mi) long and 0.5-kilometre (0.31 mi) wide Skarnsund is located in the municipality of Inderøy. On the northeastern side of the strait is the village of Vangshylla and on the southwestern side is the villages of Venneshamn and Kjerringvik (this side is the Mosvik area of Inderøy). The strait has a strong tidal current with a maelstrom.

The Skarnsund strait was crossed by the Vangshylla–Kjerringvik Ferry (car ferry), operated by Innherredsferja, prior to 19 December 1991. On that date the new 1,010-metre (3,310 ft) long Skarnsund Bridge was opened. The bridge is part of Norwegian County Road 755. Skarnsund is a noted site for both sports fishing and underwater diving.