In biology, a hypertonic cell environment has a higher concentration of solutes than the cytoplasm. In a hypertonic environment, osmosis causes water to flow out of the cell. If enough water is removed in this way, the cytoplasm can become so concentrated that the cell has difficulty functioning.

In plant cells, the effect is more dramatic. The cell membrane pulls away from the cell wall, but the cell remains joined to the adjacent cells at points called plasmodesmata . Thus, the cell takes on the appearance of a pincushion, with the plasmodesmata almost ceasing to function because they have become so constricted.

Saltwater is hypertonic to the fish that live in it. The fish cannot isolate themselves from osmotic water loss, because they need a large surface area in their gills for gas exchange. They respond by drinking large amounts of water, and excreting the salt through their kidneys.

The opposite of hypertonic is hypotonic; the intermediate state is called isotonic.