She said it was believed the attack happened at Kirkdale Cemetery at some point between 10:00 GMT and 15:30 on Thursday.
Pictures shared with the post showed two headless marble style statues either side of a headstone – which featured a picture of James and was clearly engraved with his name.
Fergus said she had been alerted by a message from a woman walking her dog and she then “rushed down” to the cemetery.
“I need to find whoever done this to my son’s resting place,” she said.
She said she tried to “help as many families as I can with either justice campaigns or helping families through the charity I set up in his memory” and she could not believe someone had done this to a baby’s grave.
She urged anyone with information to contact Merseyside Police or the charity, the James Bulger Memorial Trust.