The 150-pupil St Brigid's in Denbigh, Clwyd, will scrap tuition fees - though it will still charge fees for boarders - and will be funded by the Welsh Office from September. It will only be the 17th GM school to be established in Wales.
The Pounds 1,300-a-term school, which takes pupils aged from 13 to 18, hopes the move will enable it to expand to up to 247 pupils. New pupils will be accepted on the basic of tests taken at 11 and interviews with prospective parents.
Unlike the governing body of an LEA school, the board of directors at an independent school can become GM without having to ballot parents. However, the school says the decision had overwhelming parental support.
The headteacher of St Brigid's, Sister Elisabeth Kelly, said the move will enable the school to expand to meet local demand. "Parents would give their right arm to have their children here, as it is the only Catholic school in the area. But we are unable to accommodate them."
The decision has provoked some opposition from surrounding schools which fear it will take an unequal share of funding.
However, Welsh Office Education Minister Rod Richards said: "The governing body is convinced that maintained status will bring lasting benefits both to the school and the community. I am confident that the school will go from strength to strength while preserving its particular religious character and ethos. "