The members of the twelfth National Assembly of South Korea were elected on 12 February 1985. The Assembly sat from 11 April 1985 until 29 May 1988.[1] Their four-year term was officially supposed to be over on 10 April 1989, but the Constitution of the Sixth Republic of South Korea shortened their terms.[2]
| Date | Constituency | Incumbent | Party | New member | Party | Notes | |||
|---|---|---|---|---|---|---|---|---|---|
| 10 July 1985 | Seo, Gwangju, South Jeolla Province | Kim Nok-yeong | New Korean Democratic | Seat remained vacant | Incumbent died on 10 July 1985. No by-election held. | ||||
| 1 August 1985 | Proportional representation | Hwang In-sung | Democratic Justice | Lee Jin | Democratic Justice | Incumbent resigned on 1 August 1985 upon appointment as Minister of Agriculture; replaced by the next member in the Democratic Justice list. | |||
| Proportional representation | Kim Sung-ky | Democratic Justice | Chung Ho-keun | Democratic Justice | Incumbent resigned on 1 August 1985 upon appointment as Minister of Justice; replaced by the next member in the Democratic Justice list. | ||||
| Proportional representation | Moon Hi-gab | Democratic Justice | Kim Zoong-wie | Democratic Justice | Incumbent resigned on 1 August 1985 upon appointment as the director of the Economic Planning Board; replaced by the next member in the Democratic Justice list. | ||||
| 17 September 1985 | Proportional representation | Kim Young-jak | Democratic Justice | Park Sung-tae | Democratic Justice | Incumbent resigned on 17 September 1985; replaced by the next member in the Democratic Justice list. | |||
| 19 September 1985 | Proportional representation | Park He-kyong | Democratic Justice | Kim Chung-giun | Democratic Justice | Incumbent resigned on 19 September 1985; replaced by the next member in the Democratic Justice list. | |||
| 3 September 1986 | Proportional representation | Lee Jong-ryool | Democratic Justice | Kim Tae-soo | Democratic Justice | Incumbent resigned on 3 September 1986 upon appointment as the Chief Presidential Secretary for Public Affairs; replaced by the next member in the Democratic Justice list. | |||
| 13 October 1986 | Proportional representation | Kim Yang-bae | Democratic Justice | Hong Hee-pyo | Democratic Justice | Incumbent resigned on 13 October 1986 upon appointment as the Mayor of Gwangju; replaced by the next member in the Democratic Justice list. | |||
| 14 November 1986 | Proportional representation | Kim Chong-chul | Korean National | Cho Yong-jik | Korean National | Incumbent died on 14 November 1986; replaced by the next member in the Democratic Justice list. | |||
| 9 December 1986 | Anyang–Gwangmyeong–Siheung–Ongjin, Gyeonggi Province | Yoon Kook-ro | Democratic Justice | Seat remained vacant | Incumbent died on 9 December 1986. No by-election held. | ||||
| 27 May 1987 | Namhae–Hadong, South Gyeongsang Province | Choi Chi-whan | Korean National | Seat remained vacant | Incumbent died on 27 May 1987. No by-election held. | ||||
| 13 July 1987 | Proportional representation | Rha Woong-bae | Democratic Justice | Kim Moon-kee | Democratic Justice | Incumbent resigned on 13 July 1987 upon appointment as Minister of Commerce and Industry; replaced by the next member in the Democratic Justice list. | |||
| 24 August 1987 | Gwanak, Seoul | Yim Churl-soon | Democratic Justice | Seat remained vacant | Incumbent died on 24 August 1987. No by-election held. | ||||
| 20 November 1987 | Jongno–Jung, Seoul | Lee Min-woo | New Korean Democratic | Seat remained vacant | Incumbent resigned on 20 November 1987. No by-election held. | ||||
| 20 December 1987 | Proportional representation | Roh Tae-woo | Democratic Justice | Chung Dong-yun | Democratic Justice | Incumbent resigned on 20 December 1987 upon election as the President of South Korea; replaced by the next member in the Democratic Justice list. | |||
| 29 March 1988 | Proportional representation | Kim Hak-joon | Democratic Justice | Sim Guk-mu | Democratic Justice | Incumbent resigned on 29 March 1988; replaced by the next member in the Democratic Justice list. | |||
| 11 May 1988 | Proportional representation | Park Dong-jin | Democratic Justice | Kim Jong-yeul | Democratic Justice | Incumbent resigned on 11 May 1988 upon appointment as ambassador of South Korea to the United States; replaced by the next member in the Democratic Justice list. | |||