Bosses said they took the "hard decision", days after a male teenager died after falling 30 metres from the famous inside platform of the dome.
In a statement, they said: "Continuing to ensure the safety of all our visitors, whatever the reason they are here, is paramount and we have been reviewing this carefully over the past few days.
"This includes the safety of those on the Cathedral floor.
"We have therefore...taken the hard decision to temporarily close the Whispering Gallery whilst we assess whether there are any further safety measures that could be put in place."
Emergency services called to the historic church last Monday were unable to save the unidentified male.
St Paul's Cathedral said it was "deeply saddened" by the latest fatality.
His death - the second fatal fall at St Paul's Cathedral within the last two years - is not being treated as suspicious.
Lidia Dragescu, a 23 year old figure skater, died after falling from a Whispering Gallery in October 2017.
We are sorry to announce that we are closing the Cathedral for the rest of the day and Evensong will not take place this evening. More information to follow.
— St Paul's Cathedral (@StPaulsLondon) April 1, 2019
The cathedral insists the gallery, which has been visited by millions of people over the years, is "a safe place to visit".
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