凯尔特海（英语：Celtic Sea; 爱尔兰语：An Mhuir Cheilteach; 威尔斯语：Y Môr Celtaidd）是大西洋的一个海域，位于爱尔兰的南方。凯尔特海的东面连接着圣乔治海峡、布里斯托尔湾及英吉利海峡。另外，连接着的陆地有爱尔兰东南岸、威尔斯西南岸、英格兰康瓦耳半岛以及法国的布列塔尼半岛，全是传统上凯尔特人的聚居地，凯尔特海因而得名。本来这个海域的北部被视作圣乔治海峡的一部份，而南部并无名称，直至1921年由霍尔特提出「凯尔特海」这个名字并被广泛使用。

海域南部与西部的界线并不清晰，霍尔特提出以200㖊（366 m）等深线及乌桑特岛（Ushant）为界。国际水文组织（IHO）以铅垂线测深后把凯尔特海的定义范围再向南扩展。

The Celtic Sea (Irish:An Mhuir Cheilteach; Welsh:Y Môr Celtaidd; Cornish:An Mor Keltek; Breton:Ar Mor Keltiek; French:La mer Celtique) is the area of the Atlantic Ocean off the south coast of Ireland bounded to the east by Saint George's Channel; other limits include the Bristol Channel, the English Channel, and the Bay of Biscay, as well as adjacent portions of Wales, Cornwall, Devon, and Brittany. The southern and western boundaries are delimited by the continental shelf, which drops away sharply. The Isles of Scilly are an archipelago of small islands in the sea.