Lancaster Royal Grammar School will admit Sixth Form girls as boarders from September 2020.
This follows on from the decision to welcome girls as day pupils into Sixth Form in September 2019.
The school said a number of applications from girls for 2019 had already been received and that a coeducational Sixth Form would further enhance the quality of education offered at the school.
Dr Chris Pyle, head at LRGS, said: “I am delighted at this decision. It follows naturally from our belief that a mixed Sixth Form will give students the best possible preparation for life at university and beyond.
“Boarding at LRGS will create an exceptional opportunity for Sixth Form girls from across the region, the UK and overseas.”
Applications for boarding in 2020 will open in September . The school has around 160 boarders in total.
While many live in north-west England and the Lake District, there is also a growing number of London-based and international boarders.
The most recent Ofsted boarding inspection in February 2017 graded all aspects of boarding as Outstanding.
Inspectors praised the culture of boarding at the school, finding that that “pupils make excellent progress in their educational, moral, social and cultural development”, and that “staff are unreservedly committed to pupils’ welfare.”
Over the last six months building renovations have been taking place to create new Sixth Form study space, classrooms and a café. The school secured £2.5 m of government funding through the Priority Schools Building Project to create state of the art facilities in the Grade II listed Victorian Old School House and is due to be completed by Easter 2019.