Polywater was a hypothetical polymerized form of water that was the subject of much scientific controversy during the late 1960s.

The Soviet physicist N. N. Fedyakin had performed measurements on the properties of water that been condensed in or repeatedly forced through narrow quartz capillary tubes. Some of these experiments suggested a new form of water with a higher boiling point, lower freezing point, and much higher viscosity than ordinary water. These results were first reported to the west in 1966 when Boris Derjaguin presented Fedyakin's findings while lecturing in Great Britain.

A scientific fervor followed. Some experimentalists were able to reproduce Fedyakin's findings, while other failed. Multiple theories were advanced to explain the phenomenon, and some researchers predicted that if polywater were to contact ordinary water, it would convert that water into polywater, echoing the doomsday scenario in Kurt Vonnegut's novel Cat's Cradle.

Eventually, however, it was determined that effects observed by Fedyakin resulted from unclean capillary tubes. When subjected to chemical analysis, samples of polywater were invariably contaminated with other substances (explaining the changes in melting and boiling points), and examination of polywater via electron microscopy showed that it also contained small particles of various solids, explaining its higher viscosity.

When the experiments that had produced polywater were repeated with rigorously cleaned glassware, the anomalous properties of the resulting water vanished, and even the scientists who had originally advanced the case for polywater agreed that it did not exist.