Oral rehydration therapy (ORT) also officially known as Oral Rehydration Solution is a type of fluid replacement used to prevent and treat dehydration, especially due to diarrhea. It involves drinking water with modest amounts of sugar and salts, specifically sodium and potassium. Oral rehydration therapy can also be given by a nasogastric tube. Therapy can include the use of zinc supplements to reduce the duration of diarrhea in infants and children under the age of 5. Use of oral rehydration therapy has been estimated to decrease the risk of death from diarrhea by up to 93%.

Side effects may include vomiting, high blood sodium, or high blood potassium. If vomiting occurs, it is recommended that use be paused for 10 minutes and then gradually restarted. The recommended formulation includes sodium chloride, sodium citrate, potassium chloride, and glucose. Glucose may be replaced by sucrose and sodium citrate may be replaced by sodium bicarbonate, if not available, although the resulting mixture is not shelf stable in high-humidity environments. It works as glucose increases the uptake of sodium and thus water by the intestines, and the potassium chloride and sodium citrate help prevent hypokalemia and acidosis, respectively, which are both common side effects of diarrhea. A number of other formulations are also available including versions that can be made at home. However, the use of homemade solutions has not been well studied.

Oral rehydration therapy was developed in the 1940s using electrolyte solutions with or without glucose on an empirical basis chiefly for mild or convalescent patients, but did not come into common use for rehydration and maintenance therapy until after the discovery that glucose promoted sodium and water absorption during cholera in the 1960s. It is on the World Health Organization's List of Essential Medicines. Globally, as of 2015^[update], oral rehydration therapy is used by 41% of children with diarrhea. This use has played an important role in reducing the number of deaths in children under the age of five.

ORT is less invasive than the other strategies for fluid replacement, specifically intravenous (IV) fluid replacement. Mild to moderate dehydration in children seen in an emergency department is best treated with ORT. Persons taking ORT should eat within six hours and return to their full diet within 24–48 hours.

Oral rehydration therapy may also be used as a treatment for the symptoms of dehydration and rehydration in burns in resource-limited settings.

ORT may lower the mortality rate of diarrhea by as much as 93%. Case studies in four developing countries also have demonstrated an association between increased use of ORS and reduction in mortality. ORT using the original ORS formula has no effect on the duration of the diarrheic episode or the volume of fluid loss, although reduced osmolarity solutions have been shown to reduce stool volume.

The degree of dehydration should be assessed before initiating ORT. ORT is suitable for people who are not dehydrated and those who show signs and symptoms of mild to moderate dehydration. People who have severe dehydration should seek professional medical help immediately and receive intravenous rehydration as soon as possible to rapidly replenish fluid volume in the body.

ORT should be discontinued and fluids replaced intravenously when vomiting is protracted despite proper administration of ORT; or signs of dehydration worsen despite giving ORT; or the person is unable to drink due to a decreased level of consciousness; or there is evidence of intestinal blockage or ileus. ORT might also be contraindicated in people who are in hemodynamic shock due to impaired airway protective reflexes. Short-term vomiting is not a contraindication to receiving oral rehydration therapy. In persons who are vomiting, drinking oral rehydration solution at a slow and continuous pace will help resolve vomiting.