纪伊半岛为日本本州中部向南（太平洋侧）突出的一个半岛，是日本最大的半岛。名称来源于古代的令制国纪伊国。有一种误解的观点，认为纪伊半岛得名于纪伊国和伊势国的合称，但实际上此两者的合称为“纪势”。

广义的范围是爱知县西部(名古屋市西南部、飞岛村、弥富市和蟹江町)、三重县（员辨市北部和桑名市北部除外）、奈良县、和歌山县、大阪府中部以南(大阪市、东大阪市以南）。也有将滋贺县南部和京都府南部包括在内的说法。

狭义的范围是纪之川和栉田川连接而成的中央构造线以南地区，包括三重县的伊势志摩以南、奈良县吉野地方和整个的和歌山县。

The Kii Peninsula(紀伊半島,Kii Hantō) is the largest peninsula on the island of Honshū in Japan. It is named after the ancient Kii Province.

The area south of the “Central Tectonic Line” is called Nanki (南紀), and includes the most poleward living coral reefs in the world due to the presence of the warm Kuroshio Current, though these are threatened by global warming and human interference. Because of the Kuroshio’s strong influence, the climate of Nankii is the wettest in the Earth’s subtropics with rainfall in the southern mountains believed to reach 5 metres (200 in) per year and in the southeastern town of Owase it averages 3.85 metres (151.6 in), comparable to Ketchikan, Alaska or Tortel in southern Chile. When typhoons hit Japan the Kii Peninsula is typically the worst affected area and daily rainfalls as high as 940 millimetres (37 in) are not unknown.