Gunungsitoli (Nias pronunciation: [gu.nuŋ.si.to.li]) is a city located in North Sumatra province, Indonesia, on the Indian Ocean island of Nias, west of Sumatra. Gunungsitoli is the island's only city and is the main hub for the island and surrounding smaller islands. Located on the north-eastern side of Nias island, the city was historically a series of fortifications made by the Dutch colonial administration in the 1600s to defend against frequent raids from Nias tribes, especially those from southern parts of the island. Until 1914, it was the only part of the island that was effectively controlled by the Dutch.

Gunungsitoli had a population of 126,202 at the 2010 Census and 136,017 at the 2020 Census; in mid 2024, Gunungsitoli had a population of 145,233 (comprising 70,954 males and 74,279 females); it was officially projected to rise to 147,516 by mid 2025, and reach 150,000 in mid 2026. which makes it the seventh-most-populous city in North Sumatra. It had a population density of 309 people per square kilometre in 2024, making it the most-densely populated place on Nias island. Being the only city in the island, Gunungsitoli is the economic hub of the island and the surrounding Nias archipelago, as well as the only place with significant, non-agriculture industries, on the island. The city was previously part of the larger Nias Regency but was separated on 29 October 2008.

Nias island, together with groups of other islands off the western coast of Sumatra, was mentioned by Ptolemy in 150 CE as "Barus islands". Nias had well-established trade contacts with Arab and Chinese traders since around the seventh century. In 1154, the island was mentioned by Muhammad al-Idrisi as "Niyan" and described as "densely populated, with one big town, and inhabited by many tribes".

Archeological evidence shows humans have inhabited the island for 12,000 years. Remains of tools were found in Tögi Ndrawa cave by Indonesian archeologists from Medan in August 1999. The excavation shows sign of Mesolithic culture and that the cave was still inhabited until around 700 years ago. According to folk stories of the Nias people, the island was settled by six ancestor tribes but the current Nias people or Ono Niha—which means "human" in the Nias language—arose more recently, according to records compiled by German missionary Wilhelm Heinrich Sundermann. Migration of Ono Niha people from mainland Sumatra occurred in around 1350; they brought with them knowledge of metallurgy, agriculture, husbandry, and woven clothing. It is unknown whether previous inhabitants of the island were assimilated or out-competed with the arrival of the Ono Niha.

In 1416, Ming treasure voyages led by Zheng He occupied a portion of mainland Sumatra that directly faces Nias island and constructed a port town named Singkuang (New Land). The occupation led to a significant presence of Chinese communities on the island. Around the 1500s, the island was subject to frequent slave raids by ships from Aceh Sultanate, which at the time was under Sultan Ali Mughayat Syah, who sought to conquer the western coast of Sumatra. In 1642, seven ships from Aceh Sultanate were stranded on the eastern coast of the island, resulting in a significant presence of Acehnese communities, which are known locally as Polem people.

First contact between Nias people and Europeans came on 2 July 1664 when Dutch traders and the king of Luaha Laraga made a trade agreement and tariffs for Dutch ships that were using the port in modern-day Idanoi district. In 1668, the Dutch East India Company made agreements with village chiefs around the location of modern-day Gunungsitoli city and the Hinako islands. The company settled the region and built several warehouses but the Dutch traders left the region and abandoned the settlement in 1740 due to decreasing Dutch influence over the region.

In 1776, British traders tried to settled the region but soon also abandoned it because the region was not profitable. For several decades, there was no significant European presence on the island. The British again tried to settle the region in 1821 but the settlement was taken over by the Dutch in 1825. In 1840, the Dutch tried to gain control of the entire island following the Anglo-Dutch Treaty of 1824 but were unable to establish a military presence on the island outside of a small area of Gunungsitoli, which was then known as Rapatgebied. Frequent raids by Nias tribes against Dutch fortifications concentrated the settlement in the area that would become the city.

The Dutch were only able to start a significant military campaign against Nias tribes in 1900 and subjugated the entire island by 1914. Nias was one of the last regions of the Indonesian archipelago by to be conquered by the Dutch East Indies. Missionary activities on the island grew after the Dutch established control over the island. In 1916, a mass conversion to Christianity known as Fangesa Sebua (The Great Repentance) occurred on the island. The event started in Gunungsitoli from Idanoi and later spread throughout the island.