Emperor Gaozong of Tang (21 July 628 – 27 December 683), personal name Li Zhi, was the third emperor of the Chinese Tang dynasty, ruling from 649 to 683; after January 665, he handed power over the empire to his second wife Empress Wu (the future Wu Zetian), and her decrees were carried out with greater force than the decrees of Emperor Gaozong's. Emperor Gaozong was the youngest son of Emperor Taizong and Empress Zhangsun; his elder brothers were Li Chengqian and Li Tai.

Emperor Gaozong's reign saw the primacy of Empress Wu, who became the effective power behind the Tang rule. Empress Wu aided Emperor Gaozong in his rule during the later years of his reign after a series of strokes left him incapacitated. Emperor Gaozong effectively after January 665 delegated all matters of state to Empress Wu; after that the empress acted as the power behind the emperor, "hanging the curtain and listening to politics" (垂簾聼政; Chuílián tīngzhèng). Gaozong's personal illness, affection and trust of Wu led to her wielding a great deal of power in affairs of state until the end of his reign. From official histories from the later Tang Dynasty to contemporary historians, there exists a lively debate about whether Wu exerted undue influence on Gaozong, or whether they were equal partners in the government. Empress Wu was partially in control of power from November 660 and then totally from January 665 to December 683; there was an equality of power between Gaozong and Wu, which caused them to be called "two saints" (二聖; Er Sheng, literally two emperors) both inside and outside. After Emperor Gaozong died in December 683, power fell completely and solely into the hands of Empress Wu, acting as Empress Dowager-regent, "presiding over court and issuing edicts" (臨朝稱制; lin chao chengzhi); she subsequently became the only empress regnant in Chinese history. After his death, he was interred at the Qian Mausoleum along with Wu Zetian.

During the first part of his reign, Tang territorial gains, which started with his father Emperor Taizong, continued, including the conquest of Baekje, Goguryeo, and the Western Turkic Khaganate, but throughout the 670s, much of those gains were lost to the Tibetan Empire, Silla, Khitans, and Balhae. Further, territory previously conquered that belonged to both the Göktürks and the Western Turkic Khaganate were subjected to repeated rebellions.

Li Zhi was born in 628. He was the ninth son of his father, Emperor Taizong, and the third son of his mother, Emperor Taizong's wife Empress Zhangsun. In 631, he was named the Prince of Jin. In 633, he was made commandant of Bing Prefecture (并州, roughly modern Taiyuan, Shanxi), but remained at the capital Chang'an rather than reporting to Bing Prefecture. When Empress Zhangsun died in 636, Emperor Taizong was particularly touched by the grief that Li Zhi displayed, and from that point on particularly favored him. Sometime while he was the Prince of Jin, at the recommendation of his grand aunt Princess Tong'an, he married the grandniece of Princess Tong'an's husband Wang Yu (王裕) as his wife and princess.

Meanwhile, Li Zhi's two older brothers by Empress Zhangsun, Li Chengqian the Crown Prince and Li Tai the Prince of Wei, were locked in an intense rivalry, as Li Tai was favored by Emperor Taizong for his talent and was trying to displace Li Chengqian. Li Chengqian, in fear, entered into a conspiracy with the general Hou Junji, his uncle Li Yuanchang (李元昌) the Prince of Han, the imperial guard commander Li Anyan (李安儼), and his brothers-in-law Zhao Jie (趙節, who was also his cousin) and Du He (杜荷) to overthrow Emperor Taizong. The plot was discovered in 643, and Emperor Taizong deposed Li Chengqian. He was initially going to make Li Tai the new crown prince, but later began to believe that Li Tai's machinations were responsible for Li Chengqian's downfall. The powerful chancellor Zhangsun Wuji – Empress Zhangsun's brother—suggested that he make Li Zhi crown prince, a possibility that Li Tai was apprehensive about. Li Tai tried to intimidate Li Zhi, who had been friendly with Li Yuanchang, by pointing out to Li Zhi that Li Yuanchang had been part of the plot and that he should be concerned for himself. When Emperor Taizong noticed Li Zhi worrying about this and was told by Li Zhi of Li Tai's intimidation, Emperor Taizong's mind became set. He exiled Li Tai, and on 30 April 643, he created Li Zhi the new crown prince. He made Zhangsun and two other senior chancellors, Fang Xuanling and Xiao Yu, senior advisors to Li Zhi, and made another chancellor, Li Shiji, the head of Li Zhi's household. At the advice of another key official, Liu Ji, who pointed out that the crown prince needed to have a group of well-learned scholars that he was close to, Emperor Taizong appointed Liu, as well as Cen Wenben, Chu Suiliang, and Ma Zhou, to serve as Li Zhi's friends and advisors.

Late in 643, Emperor Taizong issued an edict to select beautiful women among good households to serve as Li Zhi's concubines. However, after Li Zhi declined such treatment, Emperor Taizong cancelled the edict. However, during his years as crown prince, he was said to have favored his concubine Pure Consort Xiao, having two daughters (the later Princesses Yiyang and Xuan Cheng) and one son (Li Sujie) with her, much to the chagrin of his wife Crown Princess Wang, who was childless and jealous of Consort Xiao. Three other concubines of his bore his other sons Li Zhong, Li Xiao (李孝), and Li Shangjin (李上金).

Around the same time, however, Emperor Taizong also became concerned that Li Zhi, who was considered kind but weak in character, would not be strong enough to be an emperor, and secretly discussed with Zhangsun Wuji the possibility of making another son by his concubine Consort Yang (daughter of Emperor Yang of Sui), Li Ke the Prince of Wu, crown prince. Zhangsun repeatedly opposed the idea, and Emperor Taizong did not carry this out.

In 645, when Emperor Taizong launched a campaign against Goguryeo, he took Li Zhi with him to Ding Prefecture (定州, roughly modern Baoding, Hebei) and then left Li Zhi there to be in charge of logistics, before heading to the front himself. He also left senior officials Gao Shilian, Liu Ji, Ma Zhou, Zhang Xingcheng, and Gao Jifu to assist Li Zhi. After the campaign ended in failure later that year, as Emperor Taizong was leading the army back from the front, Li Zhi went to meet him at Linyu Pass (臨渝關, now Shanhai Pass). Emperor Taizong suffered an injury during the campaign, and Li Zhi was said to have, as Emperor Taizong's conditions were getting worse, sucked the pus out of his wound, until Emperor Taizong recovered somewhat. In 646, with Emperor Taizong still recovering, he transferred some of the imperial authorities to Li Zhi. Li Zhi stayed at the imperial palace and attended to Emperor Taizong in his illness. That year, when Emperor Taizong was due to visit Ling Prefecture (靈州, roughly modern Yinchuan, Ningxia) to meet with a number of tribal chiefs who were formerly vassals of Xueyantuo—which had collapsed under Tang and Huige attacks earlier that year—he was set to take Li Zhi with him, but at Zhang's suggestion left Li Zhi in charge at Chang'an instead, to allow Li Zhi to become more familiar with the important affairs of state in his absence. After Emperor Taizong returned from Ling Prefecture, he retained for himself the authorities over imperial worship, state guests, military, the commissioning of officers of higher than the fifth rank, and executions, and transferred all other authorities to Li Zhi.