Whey or milk plasma is the liquid remaining after milk has been curdled and strained; it is a by-product of cheese or casein making with several commercial uses. Whey is used to produce ricotta and gjetost cheeses and is used to make many other products for human consumption and as an animal feed.

The whey protein separated from this mixture is often sold as a nutritional supplement. In addition, liquid whey contains lactose, vitamins, and minerals along with traces of fat.

As the most commonly used curdling agent is rennet, neither whey nor curds nor cheeses made therefrom automatically qualify as vegetarian foods. Vegetarians generally use vegetable-source rennet or lemon juice (or pure citric acid or sulphuric acid) to separate milk into curds and whey.

Whey is also the main constituent of the Swiss soft drink Rivella and the Austrian soft drink Lattella.