Sea worm may refer to one or several of the following taxa:
| Taxonomic name | Type | Description | Size (length) |
| Acanthocephala | phylum | parasitic worm | ranges from a few millimeters up to 65 centimetres (26 in) |
| Annelida | phylum | segmented worms | ranges from microscopic to 3 metres (9.8 ft) |
| Aplacophora | Sub-phylum | molluscs that look like worms | a few millimeters to several centimeters |
| Chaetognatha | phylum | arrow worms | 2 to 120 millimetres (0.079 to 4.724 in) |
| Cycliophora | phylum | found living attached to the bodies of lobsters | less than ½ mm wide |
| Entoprocta | phylum | sessile aquatic worms | ranges from 0.1 to 7 millimetres (0.0039 to 0.2756 in) |
| Gastrotricha | phylum | pseudocoelomate worms | 0.06 to 3 millimetres (0.0024 to 0.1181 in) |
| Gnathostomulida | phylum | jaw worms | 0.5 to 1 millimetre (0.020 to 0.039 in) |
| Hemichordata | phylum | deuterostome worms | a few centimeters to 2,5 meters |
| Kinorhyncha | phylum | pseudocoelomate invertebrates, widespread in mud or sand at all depths | 1 mm or less |
| Loricifera | phylum | sediment-dwelling worms | 100 μm to ca. 1 mm |
| Micrognathozoa | superphylum | discovered living in homothermic springs | 0.1 millimetres (0.0039 in) |
| Nematoda | phylum | round worms | ranges from microscopic to 5 centimetres (2.0 in), and some parasitic ones reaching over 1 metre (3.3 ft) |
| Nematomorpha | phylum | parasitic worms | 50 to 100 centimetres (20 to 39 in) |
| Nemertea | phylum | invertebrate ribbon worms | most are less than 20 centimetres (7.9 in) long, one specimen has been estimated at 54 metres (177 ft) |
| Phoronida | phylum | horseshoe worms | 2 centimetres (0.79 in), the largest are 50 centimetres (20 in) |
| Platyhelminthea | phylum | flatworms | |
| Priapulida | phylum | penis worms, general shape may recall the shape of a penis | 0.2 to 39 centimetres (0.079 to 15.354 in) |
| Sipuncula | class | peanut worms, a group of unsegmented marine annelids | 2 to 720 millimetres (0.079 to 28.346 in) |
| Teredinidae | family | shipworms, which are marine bivalve molluscs | several inches to five feet. |
| Xenoturbellida | subphylum | bilaterian worm-like species | up to 4 centimetres (1.6 in) |
