Mary Fagan is expected to depart by the end of the year, without a new role lined up.
A spokesman for Royal Mail said the search for a successor was 'getting under way'.
Fagan, a former Sunday Telegraph deputy City editor, said: 'After almost seven years at Royal Mail I want to look for a fresh challenge, and given that the business is beginning a whole new chapter in its development, I wanted to be open about my plan to move on.'
Royal Mail is currently overhauling its wide-ranging £2m PR roster, with most contracts for PR services due to expire later this year.
