The River Suck (IrishAn tSuca ) is a river within the Shannon River Basin in Ireland, 133 km (82.5 mi) in length. It is the main tributary of the River Shannon. It meets the Shannon a kilometre south of the village of Shannonbridge.

The river’s name is derived from the Irish suca, of uncertain meaning, possibly deriving from succínMiddle Irish for “amber.”Edmund Hogan’s Onomasticon Goedelicum records the spellings suġ (sugh), suggesting connections to Old Irish súg (“juice, sap”)

The River Suck drains an area of 1,599 square kilometres (617 sq mi). It forms much of the border between County Roscommon and County Galway, flowing along the western side of County Roscommon. Together with the Shannon on the east, it creates the long narrow form of southern County Roscommon.

The river rises in hills on the border of County Mayo and County Roscommon, and passes from Lough O’Flynn in a general south and south-easterly direction. Settlements along the river include AthleagueBallinasloeBallyforanBallymoeCastlereaGlinsk, and Tulrush, and it flows into the River Shannon a kilometre south of the village of Shannonbridge.

The water is clean and unpolluted and the river flows through unspoilt countryside with moorland, water meadows and pastureland. Some stretches are fast-flowing while others are slow and meandering. There are abundant breamrudd and tench in some parts, and perch and pike are also plentiful. To prevent flooding at Ballinasloe, a weir was erected in 1885 immediately above the four-arch bridge, with draw-doors which can be raised when there is an approaching flood.