The Blackfriars Road Post Office will be transferred to a private operator next month.
The final day of the publicly run service will be on Thursday 15 March with new operator ZCO Ltd opening for business on Monday 19 March.
Under the new operator the branch will sell cards, gifts and stationery and Saturday opening will be re-introduced.
The change means that SE1 will have just one Crown Post Office – the London Bridge branch – remaining.
ZCO already runs several post offices including the Sydenham branch.