The Qin dynasty (/tʃɪn/ CHIN) was the first imperial dynasty of China. It is named for its progenitor state of Qin, a fief of the confederal Zhou dynasty (c. 1046–256 BC). Beginning in 230 BC, the Qin under King Ying Zheng engaged in a series of wars conquering each of the rival states that had previously pledged fealty to the Zhou. This culminated in 221 BC with the successful unification of China under Qin, which then assumed an imperial prerogative – with Ying Zheng declaring himself to be Qin Shi Huang, the first emperor of China, and bringing an end to the Warring States period (c. 475–221 BC). This state of affairs lasted until 206 BC, when the dynasty collapsed in the years following Qin Shi Huang's death. The Qin dynasty's 14-year existence was the shortest of any major dynasty in Chinese history, with only two emperors. However, the succeeding Han dynasty (202 BC – 220 AD) largely continued the military and administrative practices instituted by the Qin; as a result, the Qin have been credited as the originators of the Chinese imperial system that would endure in some form until the Xinhai Revolution in 1911.

Qin was a minor power for the first several centuries of its existence; its strength greatly increased in the 4th century BC, in large part owing to the administrative and military reforms of Shang Yang. They sought to create a strong, centralised state and a large army supported by a stable economy, which were developed in the Qin homeland and implemented across China following its unification. Reforms included the standardisation of currency, weights, measures, and the writing system, along with innovations in weaponry, transportation and military tactics.

The central government sought to undercut aristocrats and landowners and administer the peasantry directly, who comprised the vast majority of the population. This enabled numerous large-scale construction projects involving the labour of hundreds of thousands of peasants and convicts – which included the connection of walls along the northern border into what would eventually become the Great Wall of China, a large national road system, and the city-sized Mausoleum of Qin Shi Huang guarded by the life-sized Terracotta Army. The state possessed an unprecedented capacity to transform the environment through the management of people and land; as a result, Qin's rise has been characterised as one of the most important events in East Asian environmental history.

When Qin Shi Huang died in 210 BC, two of his advisors placed an heir on the throne in an attempt to exert control over the dynasty and wield state power. 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 generals Xiang Yu and Liu Bang, the latter of whom founded the Han dynasty.

According to the Shiji (c. 91 BC), during the 9th century BC, Feizi – said to be a descendant of the legendary political advisor Gao Yao – was granted rule over the settlement of Qin (秦邑; modern Qingshui County, Gansu). During the rule of King Xiao of Zhou, this area became known as the state of Qin. In 897 BC, during the Gonghe Regency, the area was allocated as a dependency dedicated to raising horses. In the late 8th century BC, one of Feizi's descendants, Duke Zhuang of Qin, was summoned by the Zhou to take part in a military campaign against the Western Rong; the effort was successful and Zhuang was rewarded with additional territory. In 770 BC, Zhuang's son Duke Xiang helped escort the Zhou court under King Ping in their emergency evacuation from Fenghao to Chengzhou under threat from the Western Rong – marking the divide between the Western and Eastern Zhou periodisations. As a reward, Duke Xiang was sent as the leader of an expedition against the Western Rong to recapture the territory they had taken, during which he formally established the Qin as a major vassal state, incorporating Fenghao and much of the territory previously under direct Zhou control and thus expanding Qin eastward.

The state of Qin began military expeditions into central China in 672 BC. They initially refrained from making serious incursions due to the threat still posed by neighbouring tribes; by the 4th century BC, they had all either been subdued or conquered, setting the stage for Qin expansionism.

During the Warring States period (c. 475–221 BC), the Qin statesman Shang Yang introduced a series of advantageous military reforms between 361 BC and his death in 338. He also helped to construct the Qin capital at Xianyang (near modern Xi'an, Shaanxi) on the Wei River near the former Zhou capital of Fenghao – a city which ultimately resembled the capitals of the other states. The Qin maintained a military that was superior in both doctrine and practice to that of the other Warring States. Its army was large, efficient, and staffed with capable generals. Unlike many of their enemies, the Qin utilised contemporary advancements in weapons technology and transportation, the latter of which enabled greater mobility across the different types of terrain throughout China.

The geography of Qin's core territories – located at the heart of a region known as the Guanzhong – provided additional advantages, including fertile farmland, and a strategic position protected by mountains that made it a natural stronghold. The Guanzhong was in contrast with the flat, open Yangtze valley (also known as the "Guandong") to its south-east – during this period, Xianyang was the only capital city in China that did not require walls to be built around it. The legacy of Qin society within the Guanzhong inspired a Han-era adage that "Guanzhong produces generals, while Guandong produces ministers." The Qin's agricultural output, expanded via projects like the Wei River canal built in 246 BC, helped sustain their large army.