A Canadian gold coin named “Big Maple Leaf” and bearing the image of Queen Elizabeth II was stolen from Berlin’s Bode Museum early on Monday.
The coin is pure gold, weighs about 100kg (221lb) and has a face value of C$1m (£590,000).
“The coin was stolen last night, it’s gone,” said a museum spokesman, Markus Farr.
Given the high purity of the gold used in the coin, its material value is estimated to be as high as $4.5m (£3.58m).
The museum said on its website that the coin was issued by the Royal Canadian Mint in 2007 and was featured in the Guinness Book of Records for its “unmatched” degree of purity.
The coin, which has a diameter of 53cm (1ft 8ins) and is 3cm thick, was loaned to the Bode Museum in December 2010.
Police said it was probably stolen by a group of thieves who entered the museum undetected through a window, possibly with a ladder.
“Based on the information we have so far, we believe that the thief, maybe thieves, broke open a window in the back of the museum next to the railway tracks,” said a police spokesman, Winfrid Wenzel. “They then managed to enter the building and went to the coin exhibition.
“The coin was secured with bullet-proof glass inside the building. That much I can say,” Wenzel added.
“Neither I nor the Bode Museum can go into detail regarding personnel inside the building, the alarm system or security installations.”
The Bode Museum has one of the world’s largest coin collections, with more than 540,000 items.
The Royal Canadian Mint says on its website that it made the Big Maple Leaf “because we can”.