A by-election for the Welsh Assembly constituency of Ynys Môn was held on 1 August 2013, The by-election was triggered following the resignation on 20 June 2013 of its sitting Assembly Member, Ieuan Wyn Jones.
The election was the third Assembly by-election to be held since its formation in 1999, and the first for over seven years (the previous contest being at Blaenau Gwent in June 2006).
Prior to the by-election, the governing Labour Party held exactly half the Assembly seats, meaning a gain for that party (who have held the equivalent seat in the House of Commons since 2001) would have given the Welsh government a two-seat majority.
Election.
Six candidates were nominated for the election. 
Result.
Plaid Cymru held the seat with a greatly increased majority.
! scope="col" colspan="2" style="width: 130px"|Party
! scope="col" style="width: 17em"|Candidate
! scope="col" style="width: 5em"|Votes
! scope="col" style="width: 3.5em"|%
! scope="col" style="width: 3.5em"| ;templatestyles src="Template:Tooltip/styles.css" / ;±%
! colspan="3" style="text-align: right; margin-right: 0.5em" | Majority
! colspan="3" style="text-align: right; margin-right: 0.5em" scope=row | Turnout
! style="text-align:right;" | Swing
Previous result.
! scope="col" colspan="2" style="width: 130px"|Party
! scope="col" style="width: 17em"|Candidate
! scope="col" style="width: 5em"|Votes
! scope="col" style="width: 3.5em"|%
! scope="col" style="width: 3.5em"| ;templatestyles src="Template:Tooltip/styles.css" / ;±%
! colspan="3" style="text-align: right; margin-right: 0.5em" | Majority
! colspan="3" style="text-align: right; margin-right: 0.5em" scope=row | Turnout
! style="text-align:right;" | Swing