| Seiji Mataichi | |
| Office: | Member of the House of Councillors |
| Term Start: | 29 July 2001 |
| Term End: | 28 July 2019 |
| Constituency: | National PR |
| Office1: | Chairman of the Social Democratic Party |
| Term Start1: | 25 February 2018 |
| Term End1: | 22 February 2020 |
| Predecessor1: | Tadatomo Yoshida |
| Successor1: | Mizuho Fukushima |
| Term Start2: | 25 July 2013 |
| Term End2: | 14 October 2013 |
| Predecessor2: | Mizuho Fukushima |
| Birth Date: | 18 July 1944 |
| Birth Place: | Toyama, Japan |
| Party: | Social Democratic |
| Website: | Official website |
was a Japanese politician and previously was a member of the House of Councillors. He was also the Chairman of the Social Democratic Party. Mataichi died on 18 September 2023, at the age of 79.[1]