The River Oykel (Scottish Gaelic: Abhainn Oicill, pronounced [au.ɪɲ ɤçɡ̊ʲɪɫ̪]) is a major river in northern Scotland that is famous for its salmon fishing. It rises on Ben More Assynt, a few miles from Ullapool on the west coast of Scotland, and drains into the North Sea via the Kyle of Sutherland. Traditionally it has marked the boundary between Ross to the south and Sutherland to the north.

The river rises at a height of 1500 feet on the southern side of Ben More Assynt, and flows just over 35 miles.^{[citation needed]} The upper valley is known as Glen Oykel (Gleann Oicill) and is mostly desolate moorland; the lower valley is known as Strath Oykel (Srath Oicill). The A837 road runs along the river for most of its length, crossing it just above the junction with the Einig, about 6 miles (9.7 km) west of Rosehall at Oykel Bridge. The eponymous hotel is famous among anglers, who are obliged to stay there when fishing the river.

The River Cassley feeds into the Oykel by Invercassley (Inbhir Charsla), forming the Kyle of Sutherland which is later joined by the River Shin and the River Carron. The Kyle discharges into the North Sea via the Dornoch Firth.