Ripening is a process in fruits that causes them to become more palatable. In general, fruit becomes sweeter, less green, and softer as it ripens. Even though the acidity of fruit increases as it ripens, the higher acidity level does not make the fruit seem tarter. This effect is attributed to the Brix-Acid Ratio. Climacteric fruits ripen after harvesting and so some fruits for market are picked green (e.g. bananas and tomatoes).

Underripe fruits are also fibrous, not as juicy, and have tougher outer flesh than ripe fruits (see Mouth feel). Eating unripe fruit can sometimes lead to stomachache or stomach cramps, and ripeness affects the palatability of fruit.

Developing fruits produce compounds like alkaloids and tannins. These compounds are antifeedants, meaning that they discourage animals who would eat them while they are still ripening. This mechanism is used to make sure that fruit is not eaten before the seeds are fully developed.

At the molecular level, a variety of different plant hormones and proteins are used to create a negative feedback cycle which keeps the production of ethylene in balance as the fruit develops.

Ripening agents accelerate ripening. An important ripening agent is ethylene, a gaseous hormone produced by many plants. Many synthetic analogues of ethylene are available. They allow many fruits to be picked prior to full ripening, which is useful since ripened fruits do not ship well. For example, bananas are picked when green and artificially ripened after shipment by being exposed to ethylene. Catalytic generators are used to produce ethylene gas simply and safely. Ethylene sensors can be used to precisely control the amount of gas. Covered fruit ripening bowls or bags are commercially available. These containers increase the amount of ethylene and carbon dioxide gases around the fruit, which promotes ripening.

Calcium carbide is also used in some countries for artificially ripening fruit. When calcium carbide comes in contact with moisture, it produces acetylene gas, which is similar in its effects to the natural ripening agent, ethylene. Acetylene accelerates the ripening process. Use of Calcium carbide is illegal in most developed countries, due to contamination leading to production of the highly toxic compounds phosphine and arsine.

Climacteric fruits continue ripening after being picked, a process accelerated by ethylene gas. Non-climacteric fruits can ripen only on the plant and thus have a short shelf life if harvested when they are ripe.

Iodine (I) can be used to determine whether fruits are ripening or rotting by showing whether the starch in the fruit has turned into sugar. For example, a drop of iodine on a slightly rotten part (not the skin) of an apple will stay yellow or orange, since starch is no longer present. If the iodine is applied and takes 2–3 seconds to turn dark blue or black, then the process of ripening has begun but is not yet complete. If the iodine becomes black immediately, then most of the starch is still present at high concentrations in the sample, and hence the fruit has not fully started to ripen.