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 south entrance to Skarnsund, as seen from Vangshylla, with Skarnsund Bridge clearly visible.