"The Isis" is an alternative name for the River Thames, used from its source in the Cotswolds until it is joined by the Thame at Dorchester in Oxfordshire. It derives from the ancient name for the Thames, Tamesis, which in the Middle Ages was believed to be a combination of "Thame" and "Isis". Notably, the Isis flows through the city of Oxford.

Dorchester on Thames is a village and civil parish in Oxfordshire, about 3 miles (5 km) northwest of Wallingford and 8 miles (13 km) southeast of Oxford. The town is a few hundred yards from the confluence of the River Thames and River Thame. A common practice of the scholars at Oxford was to refer to the river Thames by two separate names, with Dorchester on Thames the point of change. Downstream of the village, the river continued to be named The Thames, while upstream it was named The Isis. Ordnance Survey maps continued the practice by labelling the river as "River Thames or Isis" above Dorchester, however, this distinction is rarely made outside the city of Oxford.