The group, which has grown from 30 to 240 people under Unwin's four-year lead, will be headed on an interim basis by deputy MD Peter Low when she departs early next month.
Unwin remains tight-lipped over her next venture, except to confirm it is in the UK media sector and is not competitive to PRN.
'It has to be exciting for me to want to leave PRN, but I have to keep it top secret until we're ready to go public,' she said.
It is understood the venture will launch in around four months time.
While Low is not named as a permanent successor, Unwin said it is likely he will remain in the role.
'There are no definite plans at the moment and we are taking stock,' said Unwin.
'The people we have on board make a successful team and there may not be one specific appointment to take my place,' she added.
Low will be supported by three client-facing directors: IR director Mark Hynes, sales and marketing director Lisa Wood and client service director Kim Burgess.
Prior to PRN, Unwin worked as director of Telegraph Enterprises, extending the Telegraph brand for commercial purposes.
She also previously ran her own financial investment consultancy, The Specialist Advisory Group on Africa, which focused on inward investment to South Africa.
At PRN, Unwin oversaw a period of aggressive expansion, including the acquisitions of NEWSdesk and Two Ten Communications in the UK, and achieving a licence to compete with the London Stock Exchange's Regulatory News Service, following the de-monopolisation of the sector last year.