Aru (كراجأن ارو; or Haru) was a major Malay Sumatran kingdom from the 13th to the 16th century. It was located on the eastern coast of North Sumatra, Indonesia. In its heyday the kingdom was a formidable maritime power, and was able to control the northern part of the Malacca strait.

The kingdom was initially established as a Melayu polity. The indigenous population practiced native animism as well as Hinduism. During the 13th century, Islam came to be practiced alongside the existing faiths. Aru's capital was located close to present-day Medan and Deli Serdang. The people of the kingdom are believed to have been descendants of the Malay from the interior of North Sumatra.

Traditionally, the location of Haru or Aru is connected to the site of its successor state, the Sultanate of Deli, which was in and around the city of Medan and Deli Serdang today, as suggested by British orientalist Winstedt. However, Groenveldt, a Dutch historian, has suggested that the center of the Haru Kingdom was located further southeast, near the estuary of the Barumun and Panai Rivers, in Labuhan Batu Regency, and is thus related and connected to the earlier Pannai Kingdom. Gilles suggested that the capital was located near Belawan Harbour, while another opinion suggested the estuary of the Wampu River by Haru Bay, Langkat Regency.

The Kota Cina site in Medan Marelan, and Benteng Putri Hijau in Deli Tua, Namorambe, Deli Serdang Regency, are archaeological sites near Medan, which are connected to the Aru Kingdom. The Benteng Putri Hijau archaeological site is under threat of residential development. Another archaeological site is Kota Rentang in Hamparan Perak area, Deli Serdang Regency, which archaeological experts have suggested was the capital of Aru Kingdom.

The earliest historical record mentioning the Haru kingdom was a Chinese chronicle dated from the Yuan dynasty (late 13th century). There is another Chinese record from a later period, the Yingya Shenglan (1416) of the Ming dynasty. The Kingdom of Haru was also mentioned in two Javanese records, the Nagarakretagama (1365) and Pararaton (c. 15th century). The Malay Annals mentioned Haru as one of a few influential kingdoms in the region, whose prestige rivaled that of Pasai and the Malacca Sultanate. The Portuguese record Suma Oriental written in the early 16th century mentioned Aru as a prosperous kingdom.

The people of Aru are believed to be the descendants of, or related to, the Karo tribe which inhabits the Tanah Karo further inland. This suggestion is based on a plausible etymology of similar sounding names, between "Aru" or "Haru" with "Karo".

The Benteng Putri Hijau archaeological site, at an ancient fortress near Medan, shows several layers of cultures estimated to date from between the 12th and 18th centuries. The fort is in the Karo and wider Batak tribes' tradition of constructing huta or kuta, a walled compound completed with parik (moat) as a defensive structure to protect the village against incessant tribal warfare. Artifacts found include stone tools, Chinese ceramics, tin bullets and Aceh coins. A fragment of lingam was discovered at the nearby site of Sukanalu, suggesting that the population of Aru adhered to both native animism and Hinduism prior to Islam entering the region.

The Haru Kingdom was mentioned in a Chinese chronicle of the Yuan dynasty. According to this source, in 1282, Kublai Khan demanded that Haru submit to China's suzerainty, and Haru responded graciously by sending tribute to Yuan's court in 1295. Marco Polo, however, did not mention Aru in his report as one of the eight kingdoms of Sumatra during his journey in 1292.