Corn starch, or cornflour, is the starch of the maize grain, commonly known as corn. It is also grown from the endosperm, or white heart, of the corn kernel. It has a distinctive appearance and feel when mixed raw with water or milk, giving easily to gentle pressure but resisting sudden pressure (see Non-Newtonian fluid). It is usually included as an anti-caking agent in powdered sugar (10X or confectioner's sugar). For this reason, recipes calling for powdered sugar often call for at least light cooking to remove the raw cornstarch taste.

The composition of cornstarch varies depending upon the feedstock, but it may be considered to be approximately 25% amylose and 75% amylopectin.

Cornstarch is often used as a binder in puddings and similar foods. Most of the packaged pudding mixes available in grocery stores include cornstarch. Cornstarch puddings can be made at home easily by using a double boiler. The most basic such pudding is made from milk, sugar, cornstarch and a flavoring agent.

Cornstarch can be used as a thickener in many recipes. Because cornstarch tends to form lumps when mixed with warm or hot water, it is best dissolved in cold water. It is also found in many gluten-free recipes.

Cornstarch also has many uses in the manufacturing of environmentally friendly products. For example, in 2004, the Japanese company Pioneer announced a biodegradable Blu-Ray disc made out of cornstarch. The use for the plastic is vast, as it is a renewable plastic that has the benefits of being biodegradable, used in injection molding, in extruders, and other common milling processes. It is also used as a medium to heavy starch for clothing.

Cornstarch has been used as a replacement for talc in baby powder.

A mixture of 1 part water to 1.5~2 parts cornstarch is a popular classroom demonstration of a dilatant (shear-thickening) fluid, often called Oobleck. When struck, cut with a knife, or worked vigorously in the hands, it behaves like a pliable solid, but if allowed to sit for a few seconds, it flows as a viscous liquid. It can also be used for making highly flammable and explosive jellies.