The Tidnish River is a short tidal river located on the Isthmus of Chignecto in Cumberland County, Nova Scotia, running along the interprovincial boundary with New Brunswick before emptying into the Northumberland Strait. Its major tributaries include the West Branch Tidnish River, Little West Branch Tidnish River, Paradise Brook, and Chapman's Brook.

The river is tidal in character, and this fundamentally shapes the paddling experience. The upper reaches past the Chignecto bridge are navigable during high tides, and paddlers are advised to time their launch accordingly. Launching at the green road bridge closest to town at the beginning of high tide allows the incoming tide to carry paddlers downstream to the boat launch near the river's mouth — a practical one-way route that takes advantage of tidal flow. Access at the green road bridge requires lifting watercraft over a guardrail and down the bank.

The river carries notable historical significance. The remains of the Chignecto Ship Railway, a 19th-century engineering project intended to haul vessels across the isthmus between the Bay of Fundy and the Northumberland Strait, are visible along the river corridor. A stone culvert over the Tidnish River and the arch bridge associated with the defunct railway are among the surviving remnants of this ambitious infrastructure project. Tidnish Dock Provincial Park, located approximately 18 kilometres northeast of Amherst, preserves remnants of the dock at the railway's northern terminus and sits near the river's mouth.

The mouth of the river supports a smelt fishery, and herring and mackerel boats are occasionally seen in the area. The downstream section from Tidnish Bridge also forms part of the interprovincial boundary between Nova Scotia and New Brunswick, and the river provides safe harbour in this lower reach.