Saga Domain (佐賀藩, Saga-han) was a Japanese domain of the Edo period Tokugawa Shogunate. It encompassed most of what are now Saga and Nagasaki Prefectures and was ruled from Saga Castle in what is now the urban center of the city of Saga. It was ruled through its history by the tozama daimyō Nabeshima clan. The domain was also referred to as Hizen Domain (肥前藩) after its location in Hizen Province or Nabeshima Domain (鍋島藩) after its ruling clan. The domain played a significant role in the Meiji Restoration.

Though the Dutch and Chinese trading posts in Nagasaki were overseen directly by officials of the Tokugawa shogunate, the domain was largely responsible for the military defense of the city and the port.

The Nabeshima clan were originally vassals of the Ryūzōji clan who controlled northern Kyushu in the Muromachi period. However, Ryūzōji Takanobu was killed in the Battle of Okitanawate with the Shimazu and Arima clans in 1584, and Nabeshima Naoshige became the guardian of Takanobu's young heir, Ryūzōji Takafusa. Six years later, Toyotomi Hideyoshi granted approval for Nabeshima to overthrow Ryūzōji and seize the territory for his own lineage. Nabeshima supported Hideyoshi's invasions of Korea in the 1590s, and fought in the Western Army, against the Tokugawa clan in the Battle of Sekigahara in 1600. During this battle, however, he turned against and captured Western Army general Tachibana Muneshige, earning some degree of favor from Tokugawa Ieyasu and being allowed to keep his fief.

The Nabeshima were allotted a kokudaka of 357,000 koku, which remained constant throughout the Tokugawa period. The Tokugawa shogunate granted the Nabeshima the ceremonial use of the "Matsudaira" surname and one kanji from the shogun's given name to all the successive daimyō, starting with Katsushige's eldest son, Nabeshima Tadanao. The domain's location close to Korea and far from Edo, the shogunal capital, along with its trade connections, brought significant foreign influence to the area. The area was a center for ceramic production and techniques as a result of its connections with Korea, becoming famous for its Imari porcelain which was a significant export good to Europe. However, although the nominal kokudaka of the domain was 357,000 koku, the actual kokudaka of the daimyō was only 60,000 koku, as the Nabeshima clan had established three sub-domains headed by cadet branches of the clan as insurance to ensure succession: Hasunoike Domain, Ogi Domain and Kashima Domain. In additional, large hatamoto stipends were granted to four branch families of Nabeshima (Shiraishi, Kawakubo, Murata, Kubota), and four branch families of the Ryūzōji (Taku, Takeo, Isahaya, Suko).

Remnants of the Ryūzōji continued to surface from time to time, however, and occasionally threatened the Nabeshima grip on power. Although members of the Ryūzōji clan ofter held high positions in domain affairs, efforts by the Ryūzōji to obtain a restoration of their status as an independent domain were denied the shogunate.

The area also bore a considerable Kirishitan population, which erupted in protest in the famous Shimabara Rebellion (1637–8).

Because the Saga Domain was close to Nagasaki, in 1631, the Shogunate ordered the domain to take turns providing troops to guard Nagasaki, exchanging on a yearly basis with Fukuoka Domain. In exchange, the domain was granted a reduction in its sankin kotai obligation, reducing the required attendance in Edo to about three months out of two years, instead of the usual two years. This was still a considerable expense, and at times the domain sought to lessen its losses by reducing the number of samurai it sent to defend the port. In 1808, during the Napoleonic Wars, the Phaeton incident occurred, in which a British frigate invaded Nagasaki and demanded the handover of the Dutch trading post. However, as Saga Domain had reduced its security personnel without permission (only 100 Saga samurai were present to deal with the situation, rather than the obligatory one thousand), the shogunate was forced to submit to the demands of the British ship, and scolded Saga harshly for its failure to fulfill its obligations. The situation surrounding the domain worsened with the death toll of the Siebold typhoon in 1828, which killed nearly 10,000 people, and the domain's finances on the brink of collapse.

The 10th daimyō, Nabeshima Naomasa (Kansō) to be informed about the Opium Wars in China, visited the Dutch warship Palembang and reaffirmed the need for Western military technology. He decided to reform the domain's administration and to adopt Western technology. In particular, he carried out extensive restructuring, reducing the number of officials by two-fifths, and devoted himself to protecting and nurturing farmers, as well as to developing and trading in industries such as pottery, tea, and coal, which improved the domain's finances. He also established organizations for the research of Western technologies, including steel refining, steam engines and artillery, and turned the domain's efforts towards these pursuits, making it one of the most modern domains in this period. Saga thus began constructing operations for the first Japanese iron refinery in 1849, and made the first use of reverberatory furnaces three years later at Tafuse by the Saga Castle in 1852. Then the domain set up Seirenkata (精煉方), the research institute for industrial technology on the site.