The 167-strong choir, based in Bangor, Gwynedd, topped the public vote on Monday evening's ITV show.
The group will now take part in Sunday's televised final - alongside dance act Entity Allstars, who also went through.
The overall winning act gets a Â£250,000 prize - and a chance to appear on the Royal Variety Performance.
The Welsh choir Cor Glanaethwy has become the first act through to the final of the Britain's Got Talent show.