David Wickes will succeed Dr Anne Cotton in a structured handover next September.
Mr Wickes said he was delighted to lead the school into the next phase of its development.
He added: ‘I feel privileged to be appointed as the next head of Portsmouth Grammar School – a school that deservedly has a national reputation.
‘My visits to the school confirmed to me that it is a wonderful community in which people work very hard and are determined to do the best that they can for the children in their care.
‘I am looking forward immensely to working with Dr Anne Cotton over the next year, ready to continue the work that is already taking place, and getting to know more people, especially the children.’
The prospective headteacher is currently in his fourth year as headmaster at Merchant Taylors’ School in Crosby, one of the leading academic day schools in the North West of England.
He has an extensive career in education, working in schools up and down the country after completing an MA in War Studies, at King’s College London.
Walter Cha, chair of governors at Portsmouth Grammar School, said: ‘David brings with him a wealth of experience leading large city day schools that, like Portsmouth Grammar School, successfully combine generations of history with a forward-looking approach to education.
‘With David’s extensive knowledge, skills and experience, he is expertly equipped to lead The Portsmouth Grammar School; to continue the excellent work started by Dr Anne Cotton, and successfully deliver the school’s strategic plan.’