Phalangium opilio is a species of harvestman belonging to the family Phalangiidae.

It is "the most widespread species of harvestman in the world", occurring natively in Europe, and much of Asia. The species has been introduced to North America, North Africa and New Zealand.

Females have a body length of 6 –, males are slightly smaller at 4 –. Males however have longer legs; the second leg is about 54 mm in males and 38 mm in females. Males and females are similarly coloured and marked, although males' markings tend to be less clear. The body has a three-lobed darker "saddle", usually with spots or dashes in the midline. Both sexes show many tubercules with small spikes on the anterior surface of their body. Mitopus morio has a very similar appearance, but P. opilio can be distinguished by the two pale "denticles" (small teeth-like structures) below the anterior margin of the carapace. Males have long forward-pointing "horns" on the second segment of their chelicerae.

Phalangium opilio is normally an univoltine species and overwinters as eggs. Eggs are laid in moist areas and hatch in three- five months. The immatures undergo several molts and reach maturity in two-three months. These harvestmans usually feed on soft-bodied animals such as aphids, caterpillars, leafhoppers, beetle larvae, mites, but sometime it may scavenge on hard-bodied animals such as various arthropods. They ares also known to feed on Helicoverpa zea eggs, and thus can act as biological pest control for soybean crops. The species is nocturnal, as is typical of opilionids.