The unnamed child was delivered at 24 weeks and could fit into a pair of cupped hands.
The record was previously held by a boy born in Germany in 2009 at 274 grams, the hospital said, citing a registry put together by the University of Iowa.
The smallest girl - also from Germany - was born at 25 weeks in 2015 weighing 0.56lbs (252g).
According to the register, only 23 babies have been born prematurely weighing less than 0.66lbs (300g) and survived, of which just four were boys.
The register collects information from medical journals, media reports and cases that have been self-reported by a family member alongside verification from a healthcare professional.
Around 90 per cent of babies born in Japan that weigh less than 2.2lbs (1,000g) survive, however, very few under 0.66lbs make it.
