A Canadian gold coin named "Big Maple Leaf" which bears the image of Queen Elizabeth II was stolen in the early hours of yesterday from Berlin's Bode Museum.
The coin is made of pure gold, weighs about 100kgs and has a face value of around $1m (€918,000).
"The coin was stolen last night, it's gone," museum spokesman Markus Farr said.
Given the high purity of the gold, the coin's material value is estimated to be $4m (€3.67m).
The museum said that the coin was issued by the Royal Canadian Mint in 2007 and that it was featured in the 'Guinness Book of Records' for its "unmatched" degree of purity.
The coin, with a diameter of 53cm and 3cm thick, was loaned to the museum in 2010.
"We believe the thief, maybe thieves, broke open a window in the back of the museum next to the railway tracks," police spokesman Winfrid Wenzel said. "They then went to the coin exhibition. The coin was secured with bullet-proof glass."
The museum has one of the world's largest coin collections with more than 540,000 items.