The Qin dynasty (/tʃɪn/) was the first dynasty of Imperial China. It is named for its progenitor state of Qin, which was a fief of the confederal Zhou dynasty that had endured for over five centuries. In 221 BC, following the completion of Qin's wars of unification conquering each of its rival states, Qin assumed an imperial prerogative under King Ying Zheng, who declared himself to be Qin Shi Huang, the first emperor of China. This state of affairs lasted until the dynasty's collapse in 206 BC.

Qin was a minor power for the early centuries of its existence. The strength of the Qin state was greatly increased by the reforms of Shang Yang in the 4th century BC, during the Warring States period. In the mid- and late 3rd century BC, the Qin state carried out a series of swift conquests, destroying the powerless Zhou dynasty and eventually conquering the other six of the Seven Warring States. Its 15-year existence was the shortest major dynasty in Chinese history, with only two emperors. Despite its short existence, the legacy of Qin strategies in military and administrative affairs shaped the consummate Han dynasty that followed, ultimately becoming seen as the originator of an imperial system that ultimately lasted in various forms until the Xinhai Revolution in 1911.

The Qin sought to create a state unified by structured centralized political power and a large military supported by a stable economy. The central government moved to undercut aristocrats and landowners to gain direct administrative control over the peasantry, who comprised the overwhelming majority of the population and labour force. This allowed ambitious projects involving three hundred thousand peasants and convicts: projects such as connecting walls along the northern border, eventually developing into the Great Wall of China, and a massive new national road system, as well as the city-sized Mausoleum of the First Qin Emperor guarded by the life-sized Terracotta Army.

The Qin introduced a range of reforms such as standardized currency, weights, measures and a uniform system of writing, which aimed to unify the state and promote commerce. Additionally, its military used the most recent weaponry, transportation and tactics, though the government was heavy-handed and bureaucratic. Qin created a system of administering people and land that greatly increased the power of the government to transform environment, and it has been argued that the subsequent impact of this system on East Asia's environments makes the rise of Qin an important event in China's environmental history.

When Qin Shi Huang died in 210 BC, two of his advisors placed an heir on the throne in an attempt to influence and control the administration of the dynasty. These advisors squabbled among themselves, resulting in both of their deaths and that of the second Qin Emperor. Popular revolt broke out and the weakened empire soon fell to Chu general Xiang Yu, who was proclaimed Hegemon-King of Western Chu, and Liu Bang, who founded the Han dynasty. Han Confucians portrayed the Qin as a monolithic, legalist tyranny, notably citing a purge known as the burning of books and burying of scholars; however, the earliest account of this event is in Sima Qian's Shiji (c. 91 BC), and some modern scholars dispute its veracity.

According to the Shiji (c. 91 BC), in the 9th century BC, Feizi, a supposed descendant of the ancient political advisor Gao Yao, was granted rule over the settlement of Qin (秦邑) (present-day Qingshui County in Shaanxi). During the rule of King Xiao of Zhou, the eighth king of the Zhou dynasty, this area became known as the state of Qin. In 897 BC, under the Gonghe Regency, the area became a dependency allotted for the purpose of raising and breeding horses. One of Feizi's descendants, Duke Zhuang, became favoured by King Ping of Zhou, the 13th king in that line. As a reward, Zhuang's son, Duke Xiang, was sent eastward as the leader of a war expedition, during which he formally established the Qin.

The state of Qin first began a military expedition into central China in 672 BC, though it did not engage in any serious incursions due to the threat from neighbouring tribesmen. By the dawn of the 4th century BC, the neighbouring tribes had all been either subdued or conquered, and the stage was set for the rise of Qin expansionism.

Lord Shang Yang, a Qin statesman of the Warring States period, introducing a number of militarily advantageous reforms from 361 BC until his death in 338 BC. Yang also helped construct the Qin capital, commencing in the mid-fourth century BC Xianyang. The resulting city greatly resembled the capitals of other Warring States.

Notably, Qin engaged in practical and ruthless warfare. During the Spring and Autumn period, the prevalent philosophy had dictated war as a gentleman's activity; military commanders were instructed to respect what they perceived to be Heaven's laws in battle. For example, when Duke Xiang of the rival state of Song was at war with the state of Chu during the Warring States period, he declined an opportunity to attack the enemy force, commanded by Zhu, while they were crossing a river. After allowing them to cross and marshal their forces, he was decisively defeated in the ensuing battle. When his advisors later admonished him for such excessive courtesy to the enemy, he retorted, "The sage does not crush the feeble, nor give the order for attack until the enemy have formed their ranks."

The Qin disregarded this military tradition, taking advantage of their enemy's weaknesses. A nobleman in the state of Wei accused the Qin state of being "avaricious, perverse, eager for profit, and without sincerity. It knows nothing about etiquette, proper relationships, and virtuous conduct, and if there be an opportunity for material gain, it will disregard its relatives as if they were animals." This, combined with a strong leadership from long-lived rulers, openness to employ talented men from other states, and little internal opposition gave the Qin a strong political base.

Another advantage of the Qin was that they had a large, efficient army and capable generals. They utilised the newest developments in weaponry and transportation as well, which many of their enemies lacked. These latter developments allowed greater mobility over several different terrain types which were most common in many regions of China. Thus, in both ideology and practice, the Qin were militarily superior.

Finally, the Qin Empire had a geographical advantage due to its fertility and strategic position, protected by mountains that made the state a natural stronghold. This was the heart of the Guanzhong region, as opposed to the Yangtze River drainage basin, known as Guandong. The warlike nature of the Qin in Guanzhong inspired a Han dynasty adage: "Guanzhong produces generals, while Guandong produces ministers." Its expanded agricultural output helped sustain Qin's large army with food and natural resources; the Wei River canal built in 246 BC was particularly significant in this respect.

During the Warring States period preceding the Qin dynasty, the major states vying for dominance were Yan, Zhao, Qi, Chu, Han, Wei and Qin. The rulers of these states styled themselves as kings, rather than using the titles of lower nobility they had previously held. However, none elevated himself to believe that he had the "Mandate of Heaven", as the Zhou kings had claimed, nor that he had the right to offer sacrifices—they left this to the Zhou rulers.

Before their conquest in the fourth and third centuries BC, the Qin suffered several setbacks. Shang Yang was executed in 338 BC by King Huiwen due to a personal grudge harboured from his youth. There was also internal strife over the Qin succession in 307 BC, which decentralised Qin authority somewhat. Qin was defeated by an alliance of the other states in 295 BC, and shortly after suffered another defeat by the state of Zhao, because the majority of their army was then defending against the Qi. The aggressive statesman Fan Sui (范雎), however, soon came to power as prime minister even as the problem of the succession was resolved, and he began an expansionist policy that had originated in Jin and Qi, which prompted the Qin to attempt to conquer the other states.

The Qin were swift in their assault on the other states. They first attacked the Han, directly east, and took their capital city of Xinzheng in 230 BC. They then struck northward; the state of Zhao surrendered in 228 BC, and the northernmost state of Yan followed, falling in 226 BC. Next, Qin armies launched assaults to the east, and later the south as well; they took the Wei city of Daliang (now called Kaifeng) in 225 BC and forced the Chu to surrender by 223 BC. Lastly, they deposed the Zhou dynasty's remnants in Luoyang and conquered the Qi, taking the city of Linzi in 221 BC.

When the conquests were complete in 221 BC, King Zheng – who had first assumed the throne of the Qin state at age 9 – became the effective ruler of China. The subjugation of the six states was done by King Zheng who had used efficient persuasion and exemplary strategy. He solidified his position as sole ruler with the abdication of his prime minister, Lü Buwei. The states made by the emperor were assigned to officials dedicated to the task rather than place the burden on people from the royal family. He then combined the titles of the earlier Three Sovereigns and Five Emperors into his new name: Shi Huangdi (始皇帝) or "First Emperor". The newly declared emperor ordered all weapons not in the possession of the Qin to be confiscated and melted down. The resulting metal was sufficient to build twelve large ornamental statues at the Qin's newly declared capital, Xianyang.

In 214 BC, Qin Shi Huang secured his boundaries to the north with a fraction (100,000 men) of his large army, and sent the majority (500,000 men) of his army south to conquer the territory of the southern tribes. Prior to the events leading to Qin dominance over China, they had gained possession of much of Sichuan to the southwest. The Qin army was unfamiliar with the jungle terrain, and it was defeated by the southern tribes' guerrilla warfare tactics with over 100,000 men lost. However, in the defeat Qin was successful in building a canal to the south, which they used heavily for supplying and reinforcing their troops during their second attack to the south. Building on these gains, the Qin armies conquered the coastal lands surrounding Guangzhou, and took the provinces of Fuzhou and Guilin. They may have struck as far south as Hanoi. After these victories in the south, Qin Shi Huang moved over 100,000 prisoners and exiles to colonize the newly conquered area. In terms of extending the boundaries of his empire, the First Emperor was extremely successful in the south.

However, while the empire at times was extended to the north, the Qin could rarely hold on to the land for long. The tribes of these locations, collectively called the Hu by the Qin, were free from Chinese rule during the majority of the dynasty. Prohibited from trading with Qin dynasty peasants, the Xiongnu tribe living in the Ordos region in northwest China often raided them instead, prompting the Qin to retaliate. After a military campaign led by General Meng Tian, the region was conquered in 215 BC and agriculture was established; the peasants, however, were discontented and later revolted. The succeeding Han dynasty also expanded into the Ordos due to overpopulation, but depleted their resources in the process. Indeed, this was true of the dynasty's borders in multiple directions; modern Xinjiang, Tibet, Manchuria, Inner Mongolia, and regions to the southeast were foreign to the Qin, and even areas over which they had military control were culturally distinct.

Three assassination attempts were made on Qin Shi Huang, leading him to become paranoid and obsessed with immortality. He died in 210 BC, while on a trip to the far eastern reaches of his empire in an attempt to procure an elixir of immortality from Taoist magicians, who claimed the elixir was stuck on an island guarded by a sea monster. The chief eunuch, Zhao Gao, and the prime minister, Li Si, hid the news of his death upon their return until they were able to alter his will to place on the throne the dead emperor's most pliable son, Huhai, who took the name of Qin Er Shi. They believed that they would be able to manipulate him to their own ends, and thus effectively control the empire. Qin Er Shi was, indeed, inept and pliable. He executed many ministers and imperial princes, continued massive building projects (one of his most extravagant projects was lacquering the city walls), enlarged the army, increased taxes, and arrested messengers who brought him bad news. As a result, men from all over China revolted, attacking officials, raising armies, and declaring themselves kings of seized territories.

During this time, Li Si and Zhao Gao fell out, and Li Si was executed. Zhao Gao decided to force Qin Er Shi to commit suicide due to Qin Er Shi's incompetence. Upon this, Ziying, a nephew of Qin Er Shi, ascended the throne, and immediately executed Zhao Gao. Ziying, seeing that increasing unrest was growing among the people and that many local officials had declared themselves kings, attempted to cling to his throne by declaring himself one king among all the others. He was undermined by his ineptitude, however, and popular revolt broke out in 209 BC. When Chu rebels under the lieutenant Liu Bang attacked, a state in such turmoil could not hold for long. Ziying was defeated near the Wei River in 207 BC and surrendered shortly after; he was executed by the Chu leader Xiang Yu. The Qin capital was destroyed the next year, and this is considered by historians to be the end of the Qin Empire. Liu Bang then betrayed and defeated Xiang Yu, declaring himself Emperor Gaozu of the new Han dynasty on 28 February 202 BC. Despite the short duration of the Qin dynasty, it was very influential on the structure of future dynasties.