Celtic Tiger (Tíogar Ceilteach) is a name for the period of rapid economic growth in the Republic of Ireland that began in the 1990s and slowed in 2001, only to pick up pace again in 2003 and then have slowed down once again by 2006. During this time, Ireland experienced a boom in which it was transformed from one of Europe's poorer countries into one of its wealthiest. The causes of Ireland's growth are the subject of some debate, but credit has been primarily given to free market capitalism: low corporate taxation; decades of investment in domestic higher education; a low-cost labour market; a policy of restraint in government spending; and EU membership - which provided transfer payments and export access to the Single Market.

The term "Celtic Tiger" has been used to refer to the country itself, and to the years associated with the boom. The first recorded use of the phrase is in a 1994 Morgan Stanley report by Kevin Gardiner. The phrase has often been wrongly associated with the Irish economist David McWilliams. The "Celtic Tiger" is analogous to the "East Asian Tigers"—the tigers of South Korea, Singapore, Hong Kong, and Taiwan during their periods of rapid tiger growth in the 1980s and 1990s. The Celtic Tiger period has also been called the "The Boom" or "Ireland's Economic Miracle". Variants of the phrase have been used to refer to continued economic growth in Ireland.

The aspiration to become a Celtic Tiger has also been expressed by leaders of the other Celtic countries. Addressing the Council on Foreign Relations in New York in October 2007, Alex Salmond, the First Minister of Scotland, said "we have everything it takes for a Celtic Lion economy to take off in Scotland" (the lion rampant is the heraldic symbol of Scotland).