"The Isis" (/ˈaɪsɪs/ EYE-siss) is an alternative name for the River Thames, used from its source in the Cotswolds until it is joined by the River Thame at Dorchester in Oxfordshire. Notably, the Isis flows through Oxford and has given its name to several institutions and products of the city.

The modern form of the name, first recorded c.1540, relates to the Egyptian goddess Isis. The deity was venerated throughout the Roman Empire, and was worshipped at the Temple of Isis near the Thames in Londinium during the Roman occupation.

The name "Isis" is especially used in the context of rowing at the University of Oxford. A number of rowing regattas are held on the Isis, including Eights Week, the most important Oxford University regatta, in the Trinity term (summer), Torpids in the Hilary term (early spring) and Christ Church Regatta for novices in the Michaelmas term (autumn). Because the width of the river is restricted at Oxford, rowing eights normally have a staggered start near Donnington Bridge and must then aim to "bump" the eight in front (i.e. catch up and touch or overlap with it sufficiently). The leading eight aims to "row over" (i.e. finish the race without being bumped).

There used to be ornate wooden barges on the river bank at the southern end of Christ Church Meadow to house rowing facilities and for viewing races. Now the barges are gone and there are boathouses instead a little further down the river near the confluence with the River Cherwell. Poplar Walk in Christ Church Meadow is used as a route to and from the boathouses.

The name "Isis" is also used for the men's second rowing eight of Oxford University Boat Club, who race against Goldie, the men's second crew of the Cambridge University Boat Club, before the annual Oxford vs. Cambridge Boat Race on the Thames in London.

The Isis, like much of the Thames, has long been popular among anglers for its freshwater fish, including trout and crayfish. The Oxford region is home to several angling clubs. W. F. Wallett, a popular Victorian clown, shares in his memoirs his own anecdote about fishing in the Isis with the celebrated circus proprietor Pablo Fanque:

For a few days I amused myself with Pablo Fanque fishing in the Isis. Pablo was a very expert angler, and would usually catch as many fish as five or six of us within sight of him put together. This suggested a curious device. You must know that Pablo is a coloured man. One of the Oxonians, with more love for angling than skill, thought there must be something captivating in the complexion of Pablo. He resolved to try. One morning, going down to the river an hour or two earlier than usual, we were astonished to find the experimental philosophic angler with his face blacked after the most approved style of the Christy's Minstrels.

The name of the river likely has Brittonic origins, influenced in its evolution by later interest in the Egyptian goddess Isis.