According to The Sunday People, the 22-year-old 'Harry Potter' star singled out after landing at New York's JFK airport and told she should be accompanied by an adult.
The flustered actress, who was eventually waved through and managed to laugh off the incident, later revealed that she is often mistaken for a kid and blamed the fact she was wearing a schoolgirl-style backpack.
According to a source, Watson was mortified, and not only did they not recognise her, they thought she was a child. It was very embarrassing but she managed to awkwardly laugh it off.
The British actress, who recently finished filming her latest movie 'This Is The End', later told of her embarrassment on Twitter.
"Passport control: 'Unaccompanied minor?' Me: 'Sorry?' Passport control: 'Where is ur guardian?' Me: 'I'm 22!!!!!!!' #neverwearingabackpackagain," the Mirror quoted her as tweeting.
"The really sad thing is that this is not the first time this has happened," she added.
The star was returning to the US where she was expected to continue her studies at Brown University, Rhode Island.