Hemolymph, or haemolymph, is a fluid, similar to the blood in vertebrates, that circulates in the inside of the arthropod's body, remaining in direct contact with the animal's tissues.

It is composed of a fluid plasma in which hemolymph cells called hemocytes are dispersed. In addition to hemocytes, the plasma also contains many chemicals. It is the major tissue type of the open circulatory system characteristic of arthropods (for example, arachnids, crustaceans and insects). In addition, some non-arthropods such as mollusks possess a hemolymphatic circulatory system.

Oxygen-transport systems were long thought unnecessary in insects, but ancestral and functional hemocyanin has been found in the hemolymph. Insect "blood" generally does not carry hemoglobin, although hemoglobin may be present in the tracheal system instead and play some role in respiration.