Donald Trump and his Japanese counterpart, Shinzo Abe, started by delicately spooning out the food into the pond to the waiting koi, which had been rounded up by a clapping Japanese aide.
The US president was then seen upending his entire wooden box into the pond.
A photograph of the moment started circulating on social media.
The incident sparked criticism among fish lovers on Twitter, with some pointing out fish cannot absorb a large amount of food at a time.
Some uncharitable Twitter users were unforgiving, with several writing: "Trump can't even feed fish right."
But footage of the incident shows it was actually Mr Abe who first upended the rest of his box into the pond.
US Secretary of State Rex Tillerson, standing just behind Mr Trump, appeared to break out in laughter at his boss's no-nonsense approach to koi feeding.
"This can make your fish sick, and excessive amounts of waste that strains the limits of what can be biologically reduced, results in a decline of water quality," the website said.
- with additional reporting from Matt Connellan
