Final answer:
In Shakespeare's play 'Julius Caesar,' a soothsayer is the first to warn Caesar of his assassination, advising him to 'beware the Ides of March.'
Step-by-step explanation:
The first person to warn Julius Caesar of his impending assassination was a soothsayer. In William Shakespeare's play Julius Caesar, the soothsayer famously warns him to 'beware the Ides of March.' This precedes the actual event on March 15, 44 BCE, where Caesar was assassinated by a group of senators. History does not record a specific individual who warned Caesar in real life, but Shakespeare's dramatization has left a lasting mark on popular culture's retelling of the event.