Hotspur Tower will be closed between November 26 and December 2 to allow repairs to the masonry on the Bondgate Within façade.
"The repairs are necessary due to a recent vehicle impact which has damaged some of the arch stones,” said a Northumberland Estates spokesperson.
"It seemed sensible to take the opportunity to get this work done during the current lockdown.
"Traffic will, of course be redirected along the usual route to the town centre.”
The 15th century gatehouse was struck by a high-sided vehicle on June 11 which caused some internal scraping and took a chunk from the top of the west-facing arch.
Afterwards, Alnwick Civic Society carried out a poll on social media to see what measures people would support to protect the tower. The most popular option was better signage (40%).
Historic England has previously warned that the building might need to be closed if further substantial damage occurred.
It was listed in its Heritage at Risk Register 2019, its annual snapshot of the state of England’s most valued historic places.
Bondgate Tower is deemed a Priority A, with Historic England stating: ‘The main defects relate to damage from high-sided vehicles passing through the central portal. Further impact of a similar nature could cause a serious collapse.’
The three-storey gatehouse was founded around 1450 by the 2nd Earl of Northumberland.
Also known as Hotspur Gate, it was originally one of four gates, in the medieval town wall.