Ringer's lactate solution (RL), also known as sodium lactate solution, Lactated Ringer’s, and Hartmann's solution, is a mixture of sodium chloride, sodium lactate, potassium chloride, and calcium chloride in water. It is used for replacing fluids and electrolytes in those who have low blood volume or low blood pressure. It may also be used to treat metabolic acidosis and to wash the eye following a chemical burn. It is given by intravenous infusion or applied to the affected area.

Side effects may include allergic reactions, high blood potassium, hypervolemia, and high blood calcium. It may not be suitable for mixing with certain medications and some recommend against use in the same infusion as a blood transfusion. Ringer's lactate solution has a lower rate of acidosis as compared with normal saline. Use is generally safe in pregnancy and breastfeeding. Ringer's lactate solution is in the crystalloid family of medications. It is isotonic, i.e. it has the same tonicity as blood.

Ringer's solution was invented in the 1880s; lactate was added in the 1930s. It is on the World Health Organization's List of Essential Medicines. Lactated Ringer's is available as a generic medication. For people with liver dysfunction, Ringer's acetate may be a better alternative with the lactate replaced by acetate. In Scandinavia Ringer's acetate is typically used.