The development will be an annex to the school for three to 18-year-olds in Ystalyfera.
Built on the site of the former Sandfields Comprehensive, the secondary school will have 650 places.
Education Secretary Kirsty Williams said: "This money will address the region's growing demand for Welsh medium education."
The Welsh Government has awarded £10.69m towards the project as part of the 21st Century Schools programme which aims to update school buildings across Wales.
Money for a new £17m Welsh medium school campus in Neath Port Talbot has been approved by the Welsh Government.