Grosnez Castle is a ruined 14th-century castle in Saint Ouen, situated in Grosnez in the north-west corner of the island of Jersey in the Channel Islands. Philippe de Carteret held it against the French when they held half of Jersey between 1461 and 1467, but it has been a ruin since the mid-16th century. Today, the ruins are open to the public. Furthermore, there are concrete steps that lead from the rear of Grosnez Castle to a small automated signal station; the station platform affords scenic views.