Ligia oceanica, the sea slater, common sea slater, or sea roach, is a woodlouse, living in the littoral zone—rocky seashores of the European North Sea and Atlantic coastlines.

L. oceanica is oval, twice as long as broad, and may reach up to 30 mm (1.2 in) in length, making it one of the largest oniscid isopods. Its colour may vary from grey to olive green, and it has large compound eyes and long antennae, two-thirds as long as its body. They are found in temperate waters from Norway to the Mediterranean Sea, and from Cape Cod north to Maine. It is a common species, occurring wherever the substrate of the littoral zone is rocky, and is especially common in crevices and rock pools and under stones. It is a nocturnal omnivore, eating many kinds of seaweed, diatoms, and detritus, with a particular fondness for bladder wrack (Fucus vesiculosus). Individuals live for 2.mw-parser-output.frac{white-space:nowrap}.mw-parser-output.frac.num,.mw-parser-output.frac.den{font-size:80%;line-height:0;vertical-align:super}.mw-parser-output.frac.den{vertical-align:sub}.mw-parser-output.sr-only{border:0;clip:rect(0,0,0,0);clip-path:polygon(0px 0px,0px 0px,0px 0px);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px}1⁄2–3 years and usually breed only once.