The extremists allegedly posted the message alongside a photo of the future king at his Battersea school on an instant messaging app.
The message included the schools address, saying 'school starts early.'
The message also featured an Arabic paragraph taken from a jihadi song, which translates: 'When war comes with the melody of bullets, we descend on disbelief, desiring retaliation.'
Residents living near Prince George's school, as well as parents of other students, had raised concerns about security at Thomas's Battersea ahead of Prince George starting classes last month.
In September, a woman was arrested at the school after trying to get into the property twice within 24 hours.
The threat against Prince George is not ISIS's first hit at the Royal Family.
In 2015, ISIS threatened the Queen in the run up to celebrations marking the 70th anniversary of the end of World War II.
Last month, extremists challenged Prince Harry to a fight in an online video.
In an English-language clip a man, identified as Abu Uqayl, said: 'Why don't you come here and fight us if you're man enough, so that we can send you and your Apaches to hellfire?'
The Royal family is yet to address the threat.
This article originally appeared on New Idea.