The Jiajing (or Chia-ching) Emperor (嘉靖 IPA: [tɕjɑ́tɕîŋ]; 16 September 1507 – 23 January 1567) was the 11th Ming Dynasty Emperor of China who ruled from 1521 to 1567. Born Zhu Houcong, he was the former Zhengde Emperor's cousin. His father, Zhu Yuyüan (1476 – 1519), the Prince of Xing, was the fourth son of the Cheng-hua emperor (1465 – 1487) and the eldest son of three sons born to the emperor's concubine, Lady Shao.

His era name means "Admirable tranquility".

As the nephew of the Hongzhi Emperor, Zhu Houcong was not brought up to succeed to the throne. However, the throne became vacant in 1521 with the sudden death of the Hongzhi Emperor's son, Emperor Zhengde, who did not leave an heir. The 14 year old Zhu Houcong was chosen to become emperor, and so relocated from his father's fief (near today's Zhongxiang, in Hubei Province) to Beijing.

As the Jiajing Emperor, Zhu Houcong had his parents posthumously elevated to an "honorary" imperial rank, and had an imperial - style Xianling Mausoleum built for them near Zhongxiang.

Custom dictated that an emperor who was not an immediate descendant of the previous one should be adopted by the previous one, to maintain an unbroken line. Such a posthumous adoption of Zhu Houcong by Emperor Zhengde was proposed, but he resisted, preferring instead to have his father declared emperor posthumously. This conflict is known as "The Great Rites Controversy." The Jiajing Emperor prevailed, and hundreds of his opponents were banished, hit in the court (廷杖) or executed. Among the banished was the great Ming poet Yang Shen.

The Jiajing Emperor was known to be a cruel and self-aggrandizing emperor and he also chose to reside outside of the Forbidden City in Beijing so he could live in isolation. Ignoring state affairs, Jiajing employed incapable individuals such as Zhang Cong and Yan Song, on whom he thoroughly relied to handle affairs of state. In time Yan Song and his son Yan Shifan - who gained power only as a result of his father's political influence - came to dominate the whole government even being called the "First and Second Prime Minister". Loyal individuals such as Hai Rui and Yang Xusheng challenged and even chastised Yan Song and his son but were thoroughly ignored by the emperor. Hai Rui and many loyal ministers were eventually dismissed or executed. Jiajing also abandoned the practice of seeing his ministers altogether from 1539 onwards and for a period of almost 25 years refused to give official audiences, choosing instead to relay his wishes through eunuchs and officials. Only Yan Song, a few handful of eunuchs and Daoist priests ever saw Jiajing. This eventually led to corruption at all levels of the Ming government.

Jiajing's ruthlessness also led to an internal plot by his concubines to assassinate him in October, 1542 by strangling him while he slept. He is said to have used young girl's menstrual blood as an elixir he hoped would give him immortality. A group of palace girls who had had enough of Jiajing's cruelty decided to band together to murder the emperor. The lead palace girl tried to strangle the emperor with ribbons from her hair while the others held down the emperor's arms and legs but made a fatal mistake by tying a knot around the emperor's neck which would not tighten. Meanwhile some of the young girls involved began to panic and one ran to the empress. The plot was exposed and on the orders of the empress, all of the girls involved, including the emperor's favourite concubine, underwent execution by the slow slicing method and all their families killed.

The Ming dynasty had enjoyed a long period of peace, but in 1542 the Mongol leader Altan Khan began to harass China along the northern border. In 1550 he even reached the suburbs of Beijing. Eventually the empire appeased him by granting special trading rights. The empire also had to deal with pirates attacking the southeastern coastline; general Qi Jiguang was instrumental in defeating these pirates.

Starting in 1550, Beijing was enlarged by adding the "Outer" or "Chinese City".

The deadliest earthquake of all times, the Shaanxi earthquake of 1556 that killed over 800,000 people, occurred during the Jiajing Emperor's reign.

In June 1548 Mongols attacked Xuan Fu and defeated the imperial army. In October they invade again and reached Hualai, which lay only a day ride from Beijing. Yen Sung blamed on the aggressive policies of his adversary Xia Yen, but he himself offered no strategy to counter the invasion. Then Xia Yen was executed. The Mongols army succeeded in penetrating the inner line of Beijing defense and in approaching to Imperial tombs.

In March 1549, the Mongols attacked Xuan Fu again, Altan routed the imperial army but the Ming force managed to block his retreat and won several encounters. During this raid, several Ming commanders were warned that if trade is not allowed, Beijing would be attacked in the fall. Upon hearing this, Jiajing Emperor ordered that measures be taken to thwart the plan.

By March 1550, no rain or snow had fallen for over 150 days. Spies reported that the hordes were assembling for a major attack. After several skirmishes near Datong in July the Mongols rode east, having been bribed by the commandant of the region to go elsewhere. On 26 September the entire raiding party breached the defense of Gubei Pass, just 40 miles northeast of Beijing and went south of Tongzhou, where a camp was established. On 30 Septembers an advance party reached the gates of Beijing and the city was besieged, and the suburbs were looted.

In 1550 the military register for capital garrisons carried about 140,000 names, but only fifty or sixty thousands were assigned to military duties, the rest worked on constructions projects. The soldiers refused to go outside of the city to fight. On the other hand, a reinforcement army arrived to defend the city but had no provisions. The army was starving, unfit to fight and was looting. The ministry of war was in stress over this situation, and on Yen Sung advise,  the various regional commanders were ordered not to pursue, so the raiding parties were able to retreat several days later.

On 6 October 1550, the minister of war was executed by Jiajing Emperor for not having been able to defend Beijing. After that, general Qiu Lan was trusted by Jiajing on military affairs, he was the commander who previously bribed the Mongols to pass by Datong. Qiu’s forces engaged the Mongols on 6 October, with thousands of menlost, and Qiu barely escaped with his life, but he reported a victory to the Emperor. Qiu lan was rewarded as commander of all army garrisons and training camps around Beijing.

He was a devoted follower of Taoism and attempted to suppress Buddhism. After the assassination attempt in 1542, Jiajing began to pay excessive attention to his Taoist pursuits while ignoring his imperial duties. He built the three Taoist temples Temple of Sun, Temple of Earth and Temple of Moon and extended the Temple of Heaven by adding the Earthly Mount. Over the years, Jiajing's devotion to Taoism was to become a heavy financial burden for the empire and create dissent across the country.

Particularly during his later years, Jiajing was known for spending a great deal of time on alchemy in hopes of finding medicines to prolong his life. He would forcibly recruit young girls in their early teens and engaged in sexual activities in hopes of empowering himself, along with the consumption of potent elixirs. He employed Taoist priests to collect rare minerals from all over the country to create elixirs, including elixirs containing mercury, which inevitably posed health problems at high doses.

After 45 years on the throne (the second longest reign in the Ming dynasty), Emperor Jiajing died in 1567 – possibly due to mercury overdose – and was succeeded by his son, the Longqing Emperor. Though his long rule gave the dynasty an era of stability, Jiajing's neglect of his official duties resulted in the decline of the dynasty at the end of the 16th century. His style of governing or for that matter the lack thereof would be emulated by his grandson later in the century.