The River Clyde (Scottish Gaelic: Abhainn Chluaidh, pronounced [ˈavɪɲ ˈxl̪ˠuəj]) is a major river of western Scotland and the third-longest in the country at about 170 kilometres (110 mi) in length. Its river network extends to 4,244 km and drains a basin of 1,903 km², expanding to 3,854 km² when the Clyde estuary system with the Kelvin, White Cart, Black Cart and Leven is included. Around 1.79 million people, 33.8% of Scotland's population, live within this catchment. The river rises in the Lowther Hills and flows north-west through South Lanarkshire and Glasgow before entering the Firth of Clyde.

From the late 18th century the upper estuary and river through Glasgow were systematically engineered using groynes, longitudinal training walls and continuous dredging, and by removing rocky obstructions such as a large part of the Elderslie Rock. This was done to enable ocean-going access and support Glasgow's rise as a world centre of shipbuilding and marine engineering in the 19th and 20th centuries.

Since the later 20th century commercial deep-water functions have migrated down the estuary to naturally deep sites at Greenock, Finnart and Hunterston, while the upper river corridor has seen major regeneration in Glasgow. Environmental quality has improved from historic industrial lows, though legacy contaminants and periodic low-oxygen episodes in the outer firth have been reported.

The earliest attested form of the name Clyde is Klōta, recorded by the 2nd-century geographer Ptolemy in his Geographia (Book II, Chapter 3, §2). The river appears as Clota in the work of the Roman historian Tacitus, referring to the Firth of Clyde in his account of Agricola's campaigns (Agricola, ch. 23). Among Brittonic-speaking inhabitants, the river was known as Clut or Clud, forms preserved in the medieval names Alt Clut ("Rock of the Clyde", i.e. Dumbarton) and Ystrad Clud ("Valley of the Clyde", Strathclyde). The modern form Clyde derives from these Brittonic names, later passing through Gaelic as Cluaidh.

Scholars such as W. J. Watson, Alan G. James, and Ranko Matasović connect the hydronym with a Common Brittonic or Proto-Celtic root Cloutā / Clōtā associated with "washing" or "cleansing"; compare the Indo-European root klū- "wash, clean", also seen in Latin cluēre "to cleanse, purify". Modern Scottish place-name authorities likewise interpret Clota as "the cleanser" or "the pure one." Watson argued in 1926 that Clota was originally the name of a river goddess personifying the Clyde. Later writers on Celtic religion, such as James MacKillop, note this as a plausible but speculative interpretation, consistent with the wider tradition of rivers in Celtic Europe being associated with female tutelary deities, however no direct inscriptions or contemporary evidence for a goddess named Clota survive.

Humans have settled along the Clyde since the Paleolithic era. Artifacts dating from 12,000 BC have been found near Biggar, a rural town close to the river. Biggar is home to an archeological site at which Britain's most ancient artifacts have been unearthed. Prehistoric canoes, used by ancient peoples for transport or trade, have been found in the river. There are a number of Mesolithic sites along the Clyde, especially in the Upper Clyde Valley. Permanent settlements and structures, including what is believed to be a temple to moon gods in Govan, were constructed in the area during the Neolithic and Bronze Ages. Celtic art, language, and other aspects of culture began spreading to the area from the south during this period, and prehistoric artifacts suggest that, by around 1000 BCE, they had become the dominant cultural influences there.

Before the legions of the Roman Empire arrived in southern Scotland, the river and the area surrounding it had been settled by the Brythonic-speaking Damnonii tribe. It has been suggested that a Damnonii town called Cathures was located there and was the precursor to modern Glasgow. The Damnonii tribe originally likely distributed power among individual chiefdoms, but at some point before 500 AD the political framework was a British culture of Welsh speakers that was politically unified and formed a centralised kingdom known as Alt Clut, representing the power centre at Dunbarton Rock.

None of the documentary or archaeological evidence from the period when the Roman legions arrived suggests that battles took place in the area. Therefore the Roman legions and Damnonii tribespeople are assumed to have been on good terms and to have co-operated by means of trade and the exchange of military information. The Romans did, however, construct several forts (castra) in the area, notably on the banks of the Clyde. These include Castledykes, Bothwellhaugh, and Old Kilpatrick and Bishopton. The Romans also constructed several roads along the river, both small ones and larger ones designed to be used as trade routes and to carry entire legions. The Antonine Wall, which lies only a few miles from the river, was constructed later by the Romans as a means of defending the area against invasion by the Picts. Despite the strategic location and flat terrain of Glasgow and the surrounding Clyde basin, no Roman civilian settlement was ever constructed. Instead, the region may have functioned as a frontier zone between the Roman province known as Britannia Inferior and the Caledonians, an indigenous group that was hostile to the Romans.