Hilo /ˈhiːloʊ/ (Hawaiian pronunciation: [ˈhilo]) is the largest settlement in and the county seat of Hawaiʻi County, Hawaii, United States, which encompasses the Island of Hawaii, and is a census-designated place (CDP). The population was 44,186 according to the 2020 census. It is the fourth-largest settlement in the state of Hawaii, the largest settlement in the state outside of Oahu, and the largest settlement in the state outside of the Greater Honolulu Area.

Hilo is in the District of South Hilo. The city overlooks Hilo Bay and has views of two shield volcanoes, Mauna Loa, an active volcano, and Mauna Kea, a dormant volcano. The Hilo bay-front has been destroyed by tsunamis twice. The majority of human settlement in Hilo stretches from Hilo Bay to Waiākea-Uka, on the flanks of the volcanoes.

Hilo is home to the University of Hawaiʻi at Hilo, ʻImiloa Astronomy Center, as well as the Merrie Monarch Festival, a week-long celebration, including three nights of competition, of ancient and modern hula that takes place annually after Easter. Hilo is also home to the Mauna Loa Macadamia Nut Corporation, one of the world's leading producers of macadamia nuts. Hilo is served by Hilo International Airport.

Around 1100 AD, the first Hilo inhabitants arrived, bringing with them Polynesian knowledge and traditions. Although archaeological evidence is scant, oral history has many references to people living in Hilo, along the Wailuku and Wailoa rivers during the time of ancient Hawaii. Oral history gives the meaning of Hilo as "to twist".

Originally, the name "Hilo" applied to a district encompassing much of the east coast of the island of Hawaiʻi, now divided into the District of South Hilo and the District of North Hilo. When William Ellis visited in 1823, the main settlement there was Waiākea on the south shore of Hilo Bay. Missionaries came to the district in the early-to-middle 19th century, founding Haili Church.

Hilo expanded as sugar plantations in the surrounding area created jobs and drew in many workers from Asia. For example, by 1887, 26,000 Chinese workers worked in Hawai'i's sugar cane plantations, one of which was the Hilo Sugar Mill. At that time, the Hilo Sugar Mill produced 3,500 tons of sugar annually.

A breakwater across Hilo Bay was begun in the first decade of the 20th century and completed in 1929. On April 1, 1946, an 8.6-magnitude earthquake near the Aleutian Islands created a 46-foot-high (14 m) tsunami that hit Hilo 4.9 hours later, killing 159 total in the islands, with 96 deaths in Hilo alone. In response, an early warning system, the Pacific Tsunami Warning Center, was established in 1949 to track these killer waves and provide warning. This tsunami also caused the end of the Hawaii Consolidated Railway, and instead the Hawaii Belt Road was built north of Hilo using some of the old railbed.

On May 22, 1960, another tsunami, caused by a 9.5-magnitude earthquake off the coast of Chile that day, claimed 61 lives, allegedly due to the failure of people to heed warning sirens. Low-lying bayfront areas of the city on Waiākea peninsula and along Hilo Bay, previously populated, were rededicated as parks and memorials.

Hilo expanded inland beginning in the 1960s. The downtown found a new role in the 1980s as the city's cultural center with several galleries and museums opening; the Palace Theater reopened in 1998 as an arthouse cinema.

Closure of the sugar plantations (including those in Hāmākua) during the 1990s hurt the local economy, coinciding with a general statewide slump. Hilo in recent years has seen commercial and population growth.

Hilo is on the eastern and windward side of the island. It is classified by the U.S. Census Bureau as a census-designated place (CDP), and has a total area of 58.3 square miles (151.0 km), 53.4 square miles (138.3 km) of which is land and 4.9 square miles (12.7 km) of which (8.4%) is water.

Hilo has a tropical rainforest climate (Köppen: Af), with substantial rainfall throughout the year. Its location on the windward coast (relative to the trade winds), makes it the fourth-wettest city in the United States, behind the southeast Alaskan cities of Whittier, Ketchikan and Yakutat, and one of the wettest in the world. An average of around 126.72 inches (3,220 mm) of rain fell at Hilo International Airport annually between 1981 and 2010, with 272 days of the year receiving some rain. Rainfall in Hilo varies with altitude, with more at higher elevations. At some weather stations in upper Hilo the annual rainfall is above 200 inches (5,100 mm).

Monthly mean temperatures range from 71.2 °F (21.8 °C) in February to 76.4 °F (24.7 °C) in August. The highest recorded temperature was 94 °F (34 °C) on May 20, 1996, and the lowest 53 °F (12 °C) on February 21, 1962. The wettest year was 1994 with 182.81 inches (4,643.4 mm), and the driest was 1983, with 68.09 inches (1,729.5 mm). The most rainfall in one month was 50.82 inches (1,290.8 mm) in December 1954. The most rainfall in 24 hours was 27.24 inches (691.9 mm) on November 2, 2000.

Hilo's location on the shore of the funnel-shaped Hilo Bay also makes it vulnerable to tsunamis.

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Historical population

Census	Pop.	Note	%±
1910	6,745		—
1920	10,431		54.6%
1930	19,468		86.6%
1940	23,353		20.0%
1950	27,198		16.5%
1960	25,966		−4.5%
1970	26,353		1.5%
1980	35,269		33.8%
1990	37,808		7.2%
2000	40,759		7.8%
2010	43,263		6.1%
2020	44,186		2.1%
source:

As of the census of 2020, 44,186 people lived in 16,225 households in the census-designated place. The population density was 796.7 inhabitants per square mile (307.6/km). The 16,905 housing units reflected an average density of 311.3 per square mile (120.2/km) in 2010 (No update on the Census for 2020).

The racial makeup was 32% Asian, 18.4% White, 10% Native Hawaiian & Pacific Islander, 0.8% African American, 0.1% American Indian & Alaska Native, 0.6% from other races, and 38.1% from two or more races. Hispanic or Latino of any race were 13% of the population.

21.1% of the households had children under the age of 18 living with them. The average household size was 2.71.

The age distribution was 21.1% under age 18, 4.5% under age 5, and 21.10% 65 or older. The percent of females are 51.1%.

The median household income on the 2020 census was $70,356, and the per capita income was $34,678. 16.1% of the population is under the poverty line.

Hilo is served by Hilo International Airport, where Hawaiian Airlines, and Southwest Airlines operate.

Hilo is served by the county Hele-On Bus.

Hilo is served by the Big Island's largest harbor, Hilo Harbor, which is on Hilo Bay.

Hilo is home to a number of educational institutions, including two post-secondary institutions, the University of Hawaiʻi at Hilo and Hawaiʻi Community College, and the Hilo and Waiakea primary and secondary school districts. Charter schools in the area serve primary and secondary students.

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Although sometimes called a city, Hilo is not an incorporated city, and does not have a municipal government. The entire island, which is between the slightly larger state of Connecticut and smaller Rhode Island in size, is under the jurisdiction of the County of Hawaiʻi, of which Hilo is the county seat. Hilo is home to county, state, and federal offices.

The oldest city in the Hawaiian archipelago, Hilo's economy was historically based on the sugar plantations of its surrounding areas, prior to their closure in the 1990s.

While Hilo has a fairly significant tourism sector, it gets less than half the annual visitors as the western coast of the Big Island, which has much sunnier weather and significantly less rain, with sandy and swimmable beaches and numerous major resorts.

A main source of tourism in Hilo is the annual week-long Merrie Monarch Festival, the world's preeminent hula competition and festival, which brings in visitors and participants from all over the world. It is held in the spring of each year beginning on Easter Sunday.

The local orchid society hosts the largest and most comprehensive orchid show in the state, the annual Hilo Orchid Show, which has been presented since 1951 and draws visitors and entrants worldwide.

Hilo is home to Hawaii's only tsunami museum, mostly dedicated to the 1946 Pacific tsunami, and is notable for the banyan trees planted by Babe Ruth, Amelia Earhart and other celebrities. It is home to the Pana'ewa Rainforest Zoo, shopping centers, cafés and other eateries, movie theaters, hotels, restaurants, and a developed downtown area with a Farmers Market. Downtown Hilo is bounded approximately by the Wailuku River, Kamehameha Avenue, Ponahawai Street, and Kapiolani Street.

The Mauna Loa Macadamia Nut Corporation is in Hilo, south of the main town off Hawaii Route 11, north of Keaʻau.

Hilo is home to most of the astronomical observatories on Mauna Kea as well as the ʻImiloa Planetarium and Museum. Astronomy has an economic impact of $100 million annually on the island. Astronomy on Mauna Kea was developed at the invitation of the Hawaii Chamber of Commerce following the collapse of the sugar cane industry.

• East Hawaiʻi Cultural Center
• Lyman House Memorial Museum
• Merrie Monarch Festival
• Pacific Tsunami Museum

Hilo is served by KWXX (94.7FM Hilo/101.5FM Kona), B93/B97 (93.1FM Kona/97.1FM Hilo), The Wave (KHBC 92.7FM Hilo), and KPUA (970AM Hilo) radio stations.

Public Access television is provided through Nā Leo TV.

The Hawaii Tribune-Herald, of Oahu Publications Inc., a subsidiary of Black Press, is Hilo's primary newspaper distribution company along with other newspapers like the Honolulu Star-Advertiser.

• Tauranga, New Zealand
• La Serena, Chile

Asteroid (342431) Hilo is named after Hilo.

Hilo also referred to the District of Hilo when the Big Island was divided into six districts by the traditional moku land division. Hilo is now divided in two: North and South Hilo Districts.

The District of North Hilo, along Hawaii State Highway 19 from north to south, encompasses the following unincorporated towns and localities:

• ʻŌʻōkala
• Laupāhoehoe and the Train Museum
• Ninole

There are locations inland along State Highway 200 including Mauna Kea mountain road, Puu Huluhulu, and others.

In the District of South Hilo, along State Highway 19, are the following unincorporated towns and localities:

• Honalo and the Akaka Falls
• Pepeekeo
• Wainaku
• Hilo Bay, the Wailuku River and the Rainbow Falls
• Hilo downtown: Pacific Tsunami Museum, etc.

Along State Highway 11 are:

• Hilo International Airport
• Prince Kuhio Plaza Shopping Center
• Pana'ewa Rainforest Zoo

There are other locations. Along State Highway 200 and its extension are:

• Kaumana
• University of Hawaii at Hilo
• Puainako Shopping Center

There are other locations.

Jasmin Iolani Hakes' 2023 book Hula: A Novel, which won Honolulu magazine's award for Book of the Year About Hawaii, is set in Hilo.

• Walter C. Dudley (2016). Tsunami!. Latitude 20. ISBN 978-0824859169.

• Pacific Tsunami Museum