No-one was injured by the falling plaster, which rained down on Thursday near Michelangelo's famed Pieta statue to the right of the main entrance.
Vatican spokesman Greg Burke said repairmen are working to secure the site and the basilica remained open.
He said it will reopen as normal on Friday.
Holy Week sees floods of pilgrims and tourists visiting St Peter's, taking part in Pope Francis's ceremonies, which are capped by Easter Sunday celebrations outside in St Peter's Square.
Reporting by Associated Press. Received via Press Association.
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