Hilo is the largest census-designated place (CDP) and the largest settlement on the , also known as the . The population was 40,759 at the 2000 census. The population increased by 6.1% to 43,263 at the 2010 census.
Hilo is the county seat of the and is located in the District of South Hilo. The town overlooks Hilo Bay, situated upon two shield volcanoes; Mauna Loa, an active volcano, and Mauna Kea, a dormant volcano and the site of the world's most important ground-based astronomical observatories. The majority of human settlement in Hilo stretches from Hilo Bay to Waiākea-Uka, on the flanks of Mauna Loa.
Hilo is home to the , , as well as the Merrie Monarch Festival, a week-long celebration of ancient and modern hula which 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. It is served by Hilo International Airport, located inside the CDP.
Circa 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.
After Kamehameha gained control of Moku O Keawe (the Hawaiian name for the ), Kamehameha celebrated the Makahiki in Hilo in 1794. The village and area of Hilo was named by Kamehameha after a special braid that was used to secure his canoe. Hilo in Hawaiian means to twist.
Kamehameha’s son, Liholiho (Kamehameha II) was born in Hilo (1797). Kamehameha’s great war fleet, Peleleu, that was instrumental in Kamehameha’s conquest, was built and based at Hilo (1796–1801). After uniting all of the islands under his rule, Hilo became Kamehameha’s first seat of government. It was in Hilo that Kamehameha established his greatest law, the Kānāwai Māmalahoe (Law of the Splintered Paddle). One of Kamehameha’s most favorite things to eat was the sweet mullet that came from Hilo’s Wailoa fish pond. Kamehameha is also known to have visited and practiced cultural protocols on Moku Ola (literally "healing island") in Hilo Bay.
Originally, the name Hilo applied to a district encompassing much of the east coast of the , now divided into the District of South Hilo and the District of North Hilo. When William Ellis visited in 1823, the main settlement in the Hilo district was Waiākea on the south shore of Hilo Bay. Missionaries came to the district in the early-to-middle 19th century, founding Haili Church, in the area of modern Hilo.
A breakwater across Hilo Bay was begun in the first decade of the 20th century and completed in 1929. On April 1, 1946, a 7.8 magnitude earthquake near the Aleutian Islands created a fourteen-meter high tsunami that hit Hilo 4.9 hours later, killing 160 people. 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 23, 1960, another tsunami, caused by a 9.5 magnitude earthquake off the coast of Chile the previous day, claimed 61 lives allegedly due to people's failure 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 being opened; the Palace Theatre was reopened in 1998 as an arthouse cinema.
Closure of the sugar plantations (including those in Hāmākua) during the 1990s led to a downturn in the local economy, coinciding with a general statewide slump. Hilo in recent years has seen commercial and population growth as the neighboring District of Puna became the fastest-growing region in the state.