| Seiji Ōsaka | |
| Office: | Deputy Leader of the Constitutional Democratic Party |
| Leader: | Kenta Izumi |
| Term Start: | 30 November 2021 |
| Predecessor: | Akira Nagatsuma |
| Office1: | Member of the House of Representatives |
| Term Start1: | 15 December 2014 |
| Constituency1: | Hokkaido-8th |
| Predecessor1: | Kazuo Maeda |
| Term Start2: | 12 September 2005 |
| Term End2: | 16 November 2012 |
| Constituency2: | Hokkaido-8th (2009–2012) Hokkaido PR (2005–2009) |
| Office3: | Mayor of Niseko |
| Term Start4: | November 1994 |
| Term End4: | 29 August 2005 |
| Predecessor4: | Tomio Watanabe |
| Successor4: | Ryūichi Satō |
| Birth Date: | 1959 4, df=yes |
| Birth Place: | Niseko, Hokkaido, Japan |
| Alma Mater: | Hokkaido University |
| Party: | CDP |
| Otherparty: | DP (2016–2017, split) DPJ (2005–2016, merger) Independent (1994–2005) |
| Nationality: | Japanese |
| Website: | Official website |
is a Japanese politician of the Constitutional Democratic Party of Japan (CDP), a member of the House of Representatives in the Diet (national legislature).[1] He represents Hokkaido's 8th district. He was a member of the Democratic Party. While he joined the CDP on the day it was formed, he ran on an independent ticket in the 2017 general election.[2]