The lugworm or sandworm (Arenicola marina) is a large marine worm of the phylum Annelida. Its coiled castings are a familiar sight on a beach at low tide but the animal itself is rarely seen except by those who, from curiosity or to use as fishing bait, dig the worm out of the sand.

Although both are visually similar and used as fishing bait, lugworms are biologically distinct from Bloodworms.

In the UK, the lugworm species Arenicola marina is commonly known as the blow lugworm, and rarely exceeds 130 millimetres (5.1 in). There is a second species of UK lugworm Arenicola defodiens commonly called the black lugworm. As well as growing larger than blow lugworm (in line with descriptions for Europe and North America lugworm below) they are generally much darker, often totally black. They can be distinguished by the different wormcasts they produce - Arenicola defodiens makes a spiral cast, while that of Arenicola marina is jumbled.

When fully grown, the lugworm of the coasts of Europe is up to 9 in (23 cm) long and 0.375 in (1 cm) in diameter. Other species on the North American coast range from 3 to 12 in (7.6 to 30.5 cm). The body is like that of any typical annelid: ringed or segmented. Its head end, which is blackish-red and bears no tentacles or bristles, passes into a fatter middle part which is red. This in turn passes into a thinner yellowish-red tail end. The middle part has bristles along its sides and also pairs of feathery gills. There is a well-developed system of blood vessels with red blood rich in the oxygen-carrying pigment, haemoglobin.