Passer-by Daniel Bevis, 22, was at the attraction with friends when he spotted the abandoned pram parked up near a bin, the Sun reports.
He said he was "horrified" to discover the tot and immediately alerted the Surrey theme park's security, who later contacted police and social services.
He said: "We were so shocked that someone could leave a kid. What's worse is that security said it happens a lot."
The buggy was found parked up against railings and a bin near the Colossus ride
The family are alleged to have ditched their child so they could queue for the ride
"We stood with her for ten to 15 minutes. No one turned up so we called security. They took her to first aid then the parents arrived. They'd been gone over an hour.
"We confronted them - a mum, dad and three other kids, who'd been on the ride - but they had nothing to say."
Thorpe Park is between the towns of Chertsey and Staines-upon-Thames
The family, thought to be from West London, is reported to have told authorities their 11-year-old daughter had been looking after the baby while they queued for the Colossus ride.
Surrey Police told the Sun: "The family's home force has been notified for a follow-up."