It has commissioned architect Sir Terry Farrell to design the high-rise scheme on around 6 acres at Mount Pleasant in Clerkenwell.
It is one of a series of UK sites the group has been looking to dispose of as it raises funds and consolidates operations ahead of a potential privatisation.
The company is already spending £32m on redeveloping part of the 12-acre site to consolidate its London operations.
A Royal Mail spokesperson said: “At the end of 2011, Camden and Islington Councils held a public consultation to explore what development might be possible on the parts of the Mount Pleasant site that Royal Mail no longer needs for its operations.
“A planning brief has now been agreed by the Councils which shows a substantial residential scheme on areas of the site that are surplus to Royal Mail’s operational requirements.”