Mount Waiʻaleʻale /ˌwaɪˌɑːleɪˈɑːleɪ/ is a shield volcano and the second highest point on the island of Kauaʻi in the Hawaiian Islands. Its name literally means "rippling water" or "overflowing water".

The mountain, at an elevation of 5,148 feet (1,569 m), averages more than 373 inches (9,500 mm) of rain a year since 1912, with a record 683 inches (17,300 mm) in 1982; its summit is one of the rainiest spots on earth. However, recent reports mention that over the period 1978–2007 the wettest spot in Hawaii is Big Bog on Maui (404 inches or 10,300 mm per year).

The summit of Waiʻaleʻale features a tropical rainforest climate (Köppen Af), with substantial rainfall throughout the course of the year. (Bodin 1978: 272) quotes 460.0 inches (11,684 mm) per year figure as being the 1912–45 average, an average that quite possibly will have changed since then, while The National Climatic Data Center quotes this figure as a 30-year average. The Weather Network and The Guinness Book of Weather Records (Holford 1977: 240) quotes 451.0 inches (11,455 mm) rain per year, while (Ahrens 2000: 528) quotes 460 inches (11,680 mm) as the average annual rainfall at Mount Waialeale and (Kroll 1995: 188) claims 510 inches (13,000 mm) falls here. Similarly, The Weather Network and the Guinness Book of Weather Records quote 335 days with rain here while (Simons 1996: 303) suggests that rain falls on 360 days per year.

The local tourist industry of Kauai has promoted it as one of the wettest places on earth, which it is. The rainfall at Waiʻaleʻale is evenly distributed through the year.

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Several factors give the summit of Waiʻaleʻale more potential to create precipitation than the rest of the island chain:

1. Its northern position relative to the main Hawaiian Islands provides more exposure to frontal systems that bring rain during the winter.
2. Its peak lies just below the so-called trade wind inversion layer of 6,000 feet (1,800 m), above which trade-wind-produced clouds cannot rise.
3. The summit plateau is flanked by steep walled valleys over 3,000 feet (910 m) deep on the three sides most consistently exposed to moisture bearing weather systems. These serve to funnel and concentrate any available precipitable water directly towards the mountain.
4. The steep cliffs of the mountain's flanks generate intense orographic lift, causing the moisture-laden air to rise rapidly – over 4,000 feet (1,200 m) in less than 0.5 miles (0.80 km) – This combined with the 'barrier' of the trade-wind inversion, serves to very efficiently squeeze almost all of the moisture out of the incoming clouds directly over and immediately downwind of the peak.

The great rainfall in the area produces the Alakaʻi Wilderness Preserve, a large boggy area that is home to many rare plants. The ground is so wet that although trails exist, access by foot to the Waiʻaleʻale area is extremely difficult.

A number of rare local plant species are named for this mountain, including Astelia waialealae, Melicope waialealae, and the endemic Dubautia waialealae.

• Big Bog, Maui

• Honolulu Star-Bulletin article on Waiʻaleʻale
• Site with hiking info on routes to Waialeale and Kawaikini and summit photos.
• "Real-time" rainfall data from the USGS Waiʻaleʻale Raingauge