Grosnez Castle, Medieval castle ruins in Saint Ouen, Jersey.

Grosnez Castle is a ruined 14th-century castle on the Grosnez headland in Saint Ouen, north-west Jersey. Its surviving remains are a gatehouse, exterior walls and four towers, all of local granite, with the walls thickest on the landward side. The gatehouse was reached by a drawbridge over a ditch cut in the bedrock and had a portcullis and doors on both south and north sides. Most of the rest is footings and wall bases exposed by 19th-century excavations.

Recent research suggests Sir Renaud de Carteret, Seigneur of St Ouen, began building the castle around 1369–70, when the Hundred Years' War resumed after the Treaty of Brétigny. It was probably unfinished when the Duc de Bourbon and Bertrand du Guesclin captured it in 1373; the French destroyed it to prevent reoccupation, and the island paid tributes until the end of 1375.

The name derives from Old Norse grar nes, "grey headland", a description of the site seen from the sea; over time the spelling shifted to resemble the French for "big nose". A nearby headland to the west, Rouge Nez, has a similar Norse-derived name. The castle is privately owned, managed by Jersey Heritage since 2003 and open to the public as a Grade 1 listed building.

Access to the ruins is maintained year-round by Jersey Heritage and available to visitors. From the site you can see clear views toward Alderney and other Channel Islands, as well as the surrounding seascapes.

The castle's name comes from Old Norse "grar nes", meaning grey headland, a description of the site as seen from the sea; the spelling later shifted to resemble the French for big nose. A nearby headland, Rouge Nez, has a similar Norse-derived name.