The Post Office in South Coast Road will move from its premises at number 138 to a unit next door to Bonnies News.
The relocation to Unit 6, at 140-144 South Coast Road, will give more space and longer opening hours.
In total it will open an extra 21 hours a week, providing access to Post Office services seven days a week.
The main Post Office hours will be extended and will be open Monday to Saturday: 9am to 5.30pm.
However, Post Office services will be available seven days a week at the Post Office serving point.
It will be open Monday to Saturday: 7am to 5.30pm and Sunday: 7am to 11am.
The branch will also open significantly earlier each day.
The date for the relocation has yet to be confirmed.
Posters will be displayed in branch to advise people of when the move will take place.
Post Office regional network manager for England South, Will Russell said: “We understand how important Post Office services are to the people of Peacehaven and we are confident this new modern Post Office service will meet the needs of the local community and secure services for the future.
“This modernisation is part of a major investment programme, the largest in the history of the Post Office and marks a commitment to no more branch closure programmes.”
The Government has committed £1.34bn for the Post Office network to make sure there is no programme of Post Office closures and to update branches; in November the Government announced a further £640m investment in the Post Office network.