The deal by the central London museum makes it the most expensive British coin ever purchased.
It depicts King Coenwulf of Mercia, who ruled Mercia and much of southern England, in the early 9th Century.
A metal detector enthusiast found it next to the River Ivel in Bedfordshire in 2001, and it was later bought by a US collector.
When the owner put it up for sale last year, the Government put a temporary export ban in place hoping it would be saved for the nation.
"The Coenwulf gold coin is tremendously significant as a new source of information on Anglo-Saxon kingship in the early ninth century"
The National Heritage Memorial Fund provided £225,000 of the £357,832 total cost.
British Museum curator Gareth Williams said they were "delighted" to have acquired the coin for the national collection.
"The Coenwulf gold coin is tremendously significant as a new source of information on Anglo-Saxon kingship in the early ninth century," he said.
Silver pennies
It is one of eight known gold coins - of which the museum now owns seven - which date back to the mid to late Saxon period.
Weighing 4.25 grams and equivalent to the value of 30 silver pennies, it is thought to be the earliest example of a gold coin in the name of an English ruler intended as part of a circulating currency.
Coenwulf ruled from 796-821 and was the most powerful single ruler in Britain at that time.
His kingdom stretched from the Thames in the south to the Humber in the north, and to the Welsh border.
The coin will go on display at the British Museum from Thursday.
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