原始凯尔特语又称凯尔特共同语，是所有已知凯尔特语言的假定祖语。虽然它是使用于西元前800年左右的祖语，但我们已经可以根据历史语言学的比较法重建其字词。原始凯尔特语是原始印欧语直接派生的女儿语，并广泛地被认为是第一支进入北欧及欧洲大西洋沿海的印欧语族语言。之后，这个语言开始在这地区从原始印欧语分化出来，变成有别于其他更早出现的颚音语组同源语的原始凯尔特语，而这地区也大致符合骨灰瓮文化西缘的哈尔施塔特文化。后来约在西元前800年之后，这个对应原始凯尔特语的哈尔施塔特文化也经由「色雷斯西米里文化」元素的影响将铁器时代带到欧洲；事实上，当时的西米里人就常从各种方面被宣称是辛布里人、苏刚布里人、及威尔斯人的祖先（见英以主义）。

The Proto-Celtic language, also called Common Celtic, is the reconstructed ancestor language of all the known Celtic languages. Its lexis can be confidently reconstructed on the basis of the comparative method of historical linguistics. Proto-Celtic is a branch of the Western Indo-European languages, along with the other branches Italic, Germanic, Hellenic, Indo-Iranian, Albanian and Balto-Slavic. The exact relationships between these branches are under discussion. The earliest archaeological culture that may justifiably be considered as Proto-Celtic is the Late Bronze Age Urnfield culture of Central Europe from the last quarter of the second millennium BC. By the Iron Age Hallstatt culture of around 800 BC, these people had become fully Celtic.