Plans for a £3.5m Welsh medium school for 11 to 14-year-olds have been backed by councillors in Caerphilly.
Caerphilly council said demand for Welsh-speaking secondary places had been rising steadily in recent years.
The school is part of a £10m plan to overhaul Welsh-medium education in the county.
Ysgol Gyfun Cwm Rhymni, in Fleur de Lys near Blackwood, is currently the area's only Welsh language school.
The council said the new school, earmarked for the old St Ilan Comprehensive School site, would feed Ysgol Gyfun Cwm Rhymni in the long term, which could be turned into a centre of excellence.
The council forecasts that there will be an over capacity of almost 1,000 Welsh language spaces at Ysgol Gyfun Cwm Rhymni by September 2020.
"It was clear that we needed an urgent solution to accommodate the increasing demand for Welsh places across the county borough," said Coun Phil Bevan, cabinet member for education.
"Our proposals mean that we can offer our children and young people an education system that is fit for the 21st Century and I'm sure parents will welcome this decision as it will benefit local pupils for many years to come."