Hong Taiji (28 November 1592 – 21 September 1643), also rendered as Huang Taiji and sometimes referred to as Abahai in Western literature, also known by his temple name as the Emperor Taizong of Qing, was the second khan of the Later Jin dynasty and the founding emperor of the Qing dynasty. He was responsible for consolidating the empire that his father Nurhaci had founded and laid the groundwork for the conquest of the Ming dynasty, although he died before this was accomplished. He conquered Inner Mongolia and the remainder of Manchuria and invaded Korea, which became a Qing tributary state. He was also responsible for changing the name of the Jurchens to "Manchu" in 1635, and changing the name of his dynasty from "Great Jin" to "Great Qing" in 1636.

It is unclear whether "Hong Taiji" was a title or a personal name. Written Hong taiji in Manchu, it was borrowed from the Mongolian title Khong Tayiji. That Mongolian term was itself derived from the Chinese huang taizi (皇太子, "crown prince", "imperial prince"), but in Mongolian it meant, among other things, something like "respected son". Alternatively, historian Pamela Crossley argues that "Hung Taiji" was a title "of Mongolian inspiration" derived from hung, a word that appeared in other Mongolian titles at the time. Early seventeenth-century Chinese and Korean sources rendered his name as "Hong Taiji" (洪台極). The modern Chinese rendering "Huang Taiji" (皇太極), which uses the character huang ("imperial"), misleadingly implies that Hong Taiji once held the title of "imperial prince" or heir apparent, even though his father and predecessor Nurhaci never designated a successor.

"Hong Taiji" was very rarely used in Manchu sources, because they observed a taboo on the personal names of emperors. In redacted documents, Hong Taiji was simply called the "Fourth Beile" or "fourth prince" (duici beile), indicating that he was the fourth ranked among the eight beile Nurhaci had designated from among his sons. Although he was the 8th prince and 4th beile amongst the Four Senior Beiles (Daisan, Amin, Manggūltai, Hong Taiji) who rotated leading administrative power on behalf of Nurhaci after the original crown prince Cuyen was deposed. At this time and after the deposition of Cuyen, Hong Taiji was awarded control of Cuyen's Plain White Banner troops by Nurhaci for his military contributions to the empire. Dudu, son of Cuyen, would control the newly split Bordered White Banner and later transferred to Ajige (Nurhaci's 12th son). After Lady Abahai was rumored to be forced to be buried with Nurhaci after his death, Hong Taiji promised to personally raise her three surviving sons Ajige, Dorgon, Dodo and in essence also controlled both White Banners for maintaining good brotherly relations. However, an archival document rediscovered in 1996 and recounting events from 1621 calls him "Hong Taiji" in a discussion concerning the possible naming of Nurhaci's heir apparent, a title that the document refers to as taise. Tatiana Pang and Giovanni Stary, two specialists of early Manchu history, consider this document as "further evidence" that Hong Taiji was his real name, "not being at all connected with the Chinese title huang taizi". Historian Mark Elliott views this as persuasive evidence that Hong Taiji was not a title, but a personal name.

Western scholars used to refer to Hong Taiji as "Abahai", but this appellation is now considered mistaken. Hong Taiji was never mentioned under this name in Manchu and Chinese sources; it was a mistake first made by Russian clergyman Vladimir Gorsky [ru] and later repeated by sinologists starting in the early twentieth century. Giovanni Stary states that this name may have originated by confusing "Abkai" with Abkai sure, which was Hong Taiji's era name in the Manchu language. Though "Abahai" is indeed "unattested in Manchu sources", it might also have derived from the Mongol word Abaġai, an honorary name given to the younger sons of hereditary monarchs. According to another view, Hong Taiji was mistakenly referred to as Abahai as a result of a confusion with the name of Nurhaci's main consort Lady Abahai.

Hong Taiji was the second Khan of the Later Jin and then Emperor of the Qing dynasty, after he changed its name. His title as Great Khan was Bogd Sécén Khaan (Manchu: Gosin Onco Hūwaliyasun Enduringge Han), and he was referred to as Bogda Khan by his Mongol subjects. His reign names, which were used in his lifetime to record dates, were Tiancong (天聰, "heavenly wisdom"; Manchu: Abka-i sure) from 1627 to 1636, and Chongde (崇德, "lofty virtue"; Manchu: Wesihun erdemungge, Mongolian: Degedü Erdemtü) from 1636 to 1643.

Hong Taiji's temple name, by which he was worshipped at the Imperial Ancestral Temple, was Taizong (太宗), the name that was conventionally given to the second emperor of a dynasty. His posthumous name, which was chosen to reflect his style of rule, was "Wen Huangdi" (文皇帝, Manchu: šu hūwangdi), which means "the culturing emperor" or "the emperor of letters".

Hong Taiji was the eighth son of Nurhaci, whom he succeeded as the second ruler of the Later Jin dynasty in 1626. He might have had Mongolian ancestry and half blood from the Yehe Nara clan as the son of Empress Xiaocigao, and he might have been genetically related to the Mongolic Daur people as a member of the Aisin-Gioro clan.

Although it has always been regarded as gossip, he was said to have been involved in the suicide of Dorgon's mother, Lady Abahai, in order to block the succession of his younger brother. This is speculated because at the time of Nurhaci's death, there were Four Senior Beiles with Hong Taiji holding the lowest rank, but also being the most fit one to rule. It was between Hong Taiji and Daisan mainly, although Manggūltai tried to thrown his name into the mix, Dorgon quickly shut him up by claiming other Jurchens and Mongolians would not support a Khan who supposedly killed his own mother. Whether it was a Lady Abahai suicide or forced death to be buried with Nurhaci after his death, the other beiles and princes would not support the rumored succession and claims of Dorgon to the throne as he and his two brothers were just teenagers without military merit. Hong Taiji was said to be fluent in Manchu, Mongolian, and the Han languages and writings. As the 4th Senior Beile with a history of military merit that matches Daisan, Hong Taiji was nice and respectful to other beiles, princes, and their children and family. Being a better strategist and politician than Daisan, Hong Taiji gained the final endorsement from Daisan and the other beiles and princes who supported Daisan turned their consolidated support towards Hong Taiji and urged him to take the throne. Hong Taiji swore he will continue love and respect all his brothers and cousins and their family. Hong Taiji also would take the throne as long as Daisan, Amin, and Manggūltai continue to share administrative power with him until he earns the power to rule alone (although this wasn't specified, it was somewhat implied). During his coronation, Hong Taiji even bowed to his 2nd brother, eldest cousin, and 5th brother as a show of honor and respect that he is elected honorably so nobody will question his succession.