The Ynys Môn by-election 2013 is a by-election that was held for the Welsh Assembly constituency of Ynys Môn on Thursday 1 August 2013, 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).