Wrexham council voted to demolish Groves High School in January - more than a decade after it closed.
The Welsh Government has confirmed the building will now be listed with "immediate effect".
Listed building consent will now be needed to demolish or alter the site.
Wrexham council leader Mark Pritchard said councillors would consider the implications of the listing in due course.
Campaigners claimed the status immediately put a halt to the planned demolition.
But they said there was still a "long road ahead" in their hopes the school would be reopened.
Lynne Williams, of Save Our Heritage Campaign Group, said the building was something the people of Wrexham could be proud of as it was the first to offer grammar school education to girls in Wales.
She said: "What has kept us going is our desire to stop the erosion of Wrexham's architectural character, which in turn impacts heavily on the appeal of the town centre.
"People simply don't want to visit or live in a bland, boring town."
Plaid Cymru AM for North Wales Llyr Gruffydd said the site should be used for one or more new schools to ease pressures.
He said: "I congratulate the Save our Heritage group who fought against all odds to stop the bulldozers and had a vision for a better use of Wrexham's remaining buildings of significance."
The decision comes after campaigners wrote to heritage body Cadw requesting the building be given special status.
The Welsh Government said the building was a key example of an inter-war girl's grammar school in the neo-classical tradition which has survived largely intact.
Economy and Infrastructure Secretary Ken Skates said the building had features displaying "stylistic flair".
He said: "These include its handsome red brick exterior, the decoration between ground floor and first floor windows with herringbone brickwork and internal quality features such as an impressive elegant double sweep staircase being flooded with natural daylight by roof lanterns.
"There is also its parquet and terrazzo flooring, the sleek timber mouldings and the main hall being well proportioned and well lit."
Campaigners fighting to save a former Wrexham school from the bulldozers have been given a lifeline after it was listed due to special architectural interest.