The River Suir (pronounced shoor) is a river in Ireland that flows into the Atlantic Ocean near Waterford after a distance of 183 kilometres.

Rising on the slopes of Borrisnoe mountain, just north of Templemore in County Tipperary, the Suir bisects the county as it flows south through towns such as Thurles and Holycross with its renowned abbey. Merging with the rivers Aherlow and Tar, it turns east at the Comeragh Mountains , forming the border between County Waterford and County Kilkenny, lying between the limestone lowlands to the north, and the Comeragh Mountains to the south. It then passes through Clonmel and Carrick-on-Suir before reaching Waterford (where it is colloquially known as the 'key river') and the estuary of Waterford Harbour as it meets the River Barrow.

Together with the River Nore and the River Barrow, the river is one of the trio known as The Three Sisters. Popular with anglers, it holds plentiful reserves of brown trout and salmon.

The Suir is known in Irish as the Siúr and it is thought the present spelling in English with the u and i reversed is due to a mistake. Some people therefore feel that the spelling in English should be Siur and this spelling is occasionally seen.

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On the side of a bridge over the Suir in Clonmel is a plaque containing the poem "Retrospect" by Charles Boland, written in 1892.

Do the feeble still venture to toddle,
to the quay and sit down on a balk,
and the Sun their old selves in the even
with the crows cawing loud in the trees
that's the spot. I think. Outside of heaven
where a heart wearied out would find ease.