厄尔尼诺现象（El Niño）或译圣婴现象，是指东太平洋海水每隔数年就会异常升温的现象，是厄尔尼诺-南方振荡现象（El Niño-Southern Oscillation，简称ENSO）中东太平洋升温的阶段，和中太平洋和东太平洋（约在国际换日线及东线120度）赤道位置产生的温暖海流有关。圣婴-南方振荡现象是指中太平洋和东太平洋赤道位置海面温度的高低温循环。厄尔尼诺现象是因为西太平洋的高气压及东太平洋的低气压所造成。圣婴-南方振荡现象中的低温阶段称为拉尼娜现象（也称为反圣婴现象），是指东太平洋的海面温度低于平均值，以及西太平洋的气压较低及东太平洋的气压较高所造成。包括圣婴现象及反圣婴现象在内的厄尔尼诺-南方振荡现象会造成全球性的温度及降雨变化。例如当圣婴现象发生时，南美洲地区会出现暴雨，而东南亚、澳大利亚则出现干旱。

El Niño /ɛl ˈniːnjoʊ/ (Spanish pronunciation: [el ˈniɲo]) is the warm phase of the El Niño Southern Oscillation (commonly called ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific (between approximately the International Date Line and 120°W), including off the Pacific coast of South America. El Niño Southern Oscillation refers to the cycle of warm and cold temperatures, as measured by sea surface temperature, SST, of the tropical central and eastern Pacific Ocean. El Niño is accompanied by high air pressure in the western Pacific and low air pressure in the eastern Pacific. The cool phase of ENSO is called "La Niña" with SST in the eastern Pacific below average and air pressures high in the eastern and low in western Pacific. The ENSO cycle, both El Niño and La Niña, causes global changes of both temperatures and rainfall. Mechanisms that cause the oscillation remain under study.