The incident took place in Barshaw Park, Paisley, at around 4pm on Friday.
The schoolboy was left with a bloody nose and facial injuries after being punched, kicked and dragged to the ground as his helpless grandmother looked on.
Police are now looking to trace the three boys responsible, who are described as being aged between 11 and 12-years-old.
A spokesperson said: “At around 4pm on Friday, we received a report that an 11-year-old boy had been assaulted by a group of youths in Barshaw Park, Paisley.
“The boy sustained minor injuries but did not require medical treatment. Enquiries are ongoing to trace those involved.
“Anyone with information that could assist with our enquiries is asked to contact police on 101.”