| Surname Nihongo: | 源 |
| Image Size: | 140px |
| Image Caption: | The emblem (mon) of the Minamoto clan, of which the Seiwa Genji were a branch |
| Parent House: | Minamoto clan |
| Founder: | Sons/grandsons of Emperor Seiwa, e.g. Minamoto no Tsunemoto (源 経基) |
| Final Ruler: | Minamoto no Sanetomo |
| Founding Year: | 10th century |
The is a line of the Japanese Minamoto clan that is descended from Emperor Seiwa, which is the most successful and powerful line of the clan. Many of the most famous Minamoto members, including Minamoto no Yoshitsune, Minamoto no Yoritomo, the founder of the Kamakura shogunate; and Ashikaga Takauji, the founder of the Ashikaga shogunate, belonged to this line. Tokugawa Ieyasu (1543–1616), founder of the Tokugawa shogunate, also claimed descent from this lineage. The family is named after Emperor Seiwa, whose four sons and twelve grandsons founded the Seiwa Genji. Emperor Seiwa was father of Imperial Prince Sadazumi (貞純親王 Sadazumi Shinnō) (873–916), who was in turn the father of Minamoto no Tsunemoto (源経基) (894–961), one of the founders of the Seiwa Genji, from whom most Seiwa Genji members are descended. Many samurai families belong to this line and used "Minamoto" clan name in official records, such as the Ashikaga, Hatakeyama, Hosokawa, Imagawa, Mori, Nanbu, Nitta, Ogasawara, Ōta, Satake, Satomi, Shiba, Takeda, Toki and the Tsuchiya, among others. The Shimazu and Tokugawa clans also claimed to belong to this line.
A group of Shinto shrines connected closely with the clan is known as the Three Genji Shrines (源氏三神社 Genji San Jinja).
The following family trees are a non-exhaustive listing of the Seiwa Genji and the clans that branched from the family.
Legend: Solid lines represent blood relationship; dashed lines represent adoptions. An asterisk indicates a monk (who would not have been allowed to retain the Minamoto name).