Sō clan (宗氏, Sō-shi) was a Japanese clan claiming descent from Taira no Tomomori. The clan governed and held Tsushima Island from the 13th through the late 19th century, from the Kamakura period until the end of the Edo period and the Meiji Restoration.

In 1587, Toyotomi Hideyoshi confirmed the clan's possession of Tsushima. In the struggles which followed Hideyoshi's death, the clan sided with the Tokugawa; however, they did not participate in the decisive battles which preceded the establishment of the Tokugawa shogunate. The descendants of tozama Sō Yoshitoshi (1568–1615) remained at Tsushima-Fuchū Domain (100,000 koku) in Tsushima Province until the abolition of the han system. The head of this clan line was ennobled as count in 1884.

Historians consider the Sō clan to have been an offshoot of the Koremune clan, who served as local officials of Dazaifu and Tsushima Province. The earliest evidence of Sō clan cohesion arises in the 11th century. The Koremune had their start as governors of Tsushima following an incident in 1246, when the Abiru clan, local district officials (zaichōkanjin) on Tsushima, rebelled against the Chinzei Bugyō and the Dazaifu government that governed all of Kyūshū and the surrounding regions on behalf of the Kamakura shogunate. Dazaifu ordered Koremune Shigehisa to stop the rebellion and to destroy the Abiru clan. The Shōni clan, the shugo 'governors' of Tsushima, rewarded him for his victory with the post of jitō 'local land steward'.

The Koremune extended its influence on Tsushima over the course of the Kamakura period, as the deputies of the Shōni. When the Mongols invaded Japan in 1274, clan head Sō Sukekuni fought against the invaders and died on Tsushima. The Sō clan fought for the Shōni clan and for the Ashikaga's Northern Court during the Nanboku-chō period (1336–1392), and seized a portion of Chikuzen Province. Imagawa Ryōshun became Chinzei Tandai (head of the Dazaifu government) soon afterwards, and the Imagawa clan became shugo of Tsushima. When Imagawa Ryōshun was dismissed from his post in 1395, Sō Sumishige became shugo of Tsushima.

Though now holding the hereditary post of shugo of Tsushima, the clan remained vassals of the Shōni until the late 15th century. In the course of breaking away from the Shōni clan, the Sō clan started to claim that it originated with a grandson of Taira no Tomomori, Taira no Tomomune. The Sō clan moved its base from northern Kyushu to Tsushima around 1408. Although it struggled to keep its territory in Chikuzen on Kyushu, the clan was finally purged from that region by the Ōuchi clan in the mid-15th century.

Alongside the Shōni clan, whose hereditary clan heads now regularly operated under Sō clan guidance, the Sō fought the Ōuchi numerous times across the Sengoku period (1467–1600), and later the Mōri and Ōtomo clans as well; the clan lost and regained their territory in Chikuzen province on Kyushu many times over the course of the period. In the end, the downfall of the Shōni, marked by Shōni Fuyuhisa's 1559 defeat at the hands of Ryūzōji Takanobu, brought an end to the Sō clan's territorial aspirations on Kyushu.

Following a period of increased wokou predation, the Joseon-Japanese "Treaty of Tenbun" in 1547 (Tenbun year 11) limited trading to the Joseon port of Busan and also limited Sō clan commerce to 20 ships annually.

The Sō clan submitted to Toyotomi Hideyoshi in 1587 and supplied troops for the invasions of Korea during the Imjin War. The Sō sided with the Western Army of Ishida Mitsunari at the Battle of Sekigahara (1600), yet they were not punished by Tokugawa Ieyasu. The prevalent theory is that Ieyasu sought to improve relations with Korea and China, and therefore pardoned the Sō clan, which had a diplomatic channel with Korea.