Former President Bill Clinton used the collector's item to swear in de Blasio Wednesday, but hours after the ceremony the Bible -- which had been given to Roosevelt as a gift at his first inauguration in March 1933 -- was nowhere to be seen, the New York Post reported.
About 50 police officers and city officials searched for the Bible at City Hall, but were unable to find it by Wednesday afternoon.
"They had the whole detail looking through blankets," a police officer said.
A police detective found the book just before 5 p.m., shouting "we found it," prompting applause from a crowd that had been searching the building.
The FDR Bible came from a collection held at the FDR Presidential Library in Hyde Park, N.Y.
Library director Lynn Bassanese, who loaned the Bible to de Blasio, said FDR had signed it.
"Since FDR signed it and dated it for his very first inauguration, we thought that was particularly symbolic for the mayor-elect," she said.
The newspaper did not say where the police detective found the Bible and there was no word on its condition once it was found.