Heqin, also known as marriage alliance, refers to the historical practice of Chinese monarchs marrying princesses—usually members of minor branches of the ruling family—to rulers of neighboring states. It was often adopted as an appeasement strategy with an enemy state that was too powerful to defeat on the battlefield. The policy was not always effective. It implied an equal diplomatic status between the two monarchs. As a result, it was controversial and had many critics.

Lou Jing, the author of the policy, proposed granting the eldest daughter of Emperor Gaozu of Han to Modu Chanyu of the Xiongnu. His proposal was adopted and implemented with a treaty in 198 BC, following the Battle of Baideng two years prior. Wang Zhaojun, of the Han dynasty, and Princess Wencheng, of the Tang dynasty, are among the most famous heqin princesses.

The 20th-century scholar Wang Tonglin praised heqin for facilitating the "melting of races" in China.

There were a total of fifteen instances of heqin marriage alliances during the Han dynasty.

The Han dynasty sent random unrelated commoner women falsely labeled as "princesses" and members of the Han imperial family multiple times when they were practicing Heqin marriage alliances with the Xiongnu in order to avoid sending the emperor's daughters.

The Xiongnu practiced marriage alliances with Han dynasty officers and officials by marrying off daughters of the Chanyu (the Xiongnu ruler) to Han people who joined the Xiongnu and Xiongnu in Han service. The daughter of the Laoshang Chanyu (and older sister of Junchen Chanyu and Yizhixie Chanyu) was married to the Xiongnu General Zhao Xin, the Marquis of Xi who was serving the Han dynasty. The daughter of the Qiedihou Chanyu was married to the Han general Li Ling after he surrendered and defected. The Yenisei Kirghiz Khagans claimed descent from Li Ling. Another Han general who defected to the Xiongnu was Li Guangli who also married a daughter of the Hulugu Chanyu. The Han diplomat Su Wu married a Xiongnu woman given by Li Ling when he was arrested and taken captive. The Han explorer Zhang Qian married a Xiongnu woman and had a child with her when he was taken captive by the Xiongnu. The Emperor Wu of Han dispatched Zhang Qian to explore the Western Regions and to form an alliance with the Yuezhi people in order to combat the Xiongnu. During this time Zhang married a Xiongnu wife, who bore him a son, and gained the trust of the Xiongnu leader.

The Yenisei Kyrgyz khagans of the Yenisei Kyrgyz Khaganate claimed descent from the Han general Li Ling, grandson of the famous general Li Guang. Li Ling was captured by the Xiongnu and defected in the first century BCE. And since the Tang imperial Li family also claimed descent from Li Guang, the Kirghiz Khagan was therefore recognized as a member of the Tang imperial family. This relationship soothed the relationship when Kyrgyz khagan Are (阿熱) invaded Uyghur Khaganate and put Qasar Qaghan to the sword. The news brought to Chang'an by Kyrgyz ambassador Zhuwu Hesu (註吾合素).

Ban Zhi married a Xiongnu Jin royal woman from the Xiutu royal family