This is a list of by-elections to the Senedd (Welsh Parliament; Welsh: Senedd Cymru; formerly known as the National Assembly for Wales).
| Assembly term | By-election | Date | Incumbent | Party | Winner | Party | Cause | Retained at next election | ||
|---|---|---|---|---|---|---|---|---|---|---|
| 2016–2021 | Alyn and Deeside | 6 February 2018 | Carl Sargeant | Labour | Jack Sargeant | Labour | Death (suicide)[1] | Yes | ||
| 2011–2016 | Ynys Môn | 1 August 2013 | Ieuan Wyn Jones | Plaid Cymru | Rhun ap Iorwerth | Plaid Cymru | Resignation to lead the Menai Science Park | Yes | ||
| 2003–2007 | Blaenau Gwent | 29 June 2006 | Peter Law* | Labour | Trish Law | Independent | Death (brain tumour) | Yes | ||
| 1999–2003 | Swansea East | 27 September 2001 | Val Feld | Labour | Valerie Lloyd | Labour | Death (cancer) | Yes | ||
*Peter Law left the Labour Party and sat as an independent prior to his death. The by-election was won by his widow, but is classed as an independent gain from Labour.