A TEENAGE boy is in a serious condition in hospital after he was stabbed by a 13-year-old girl outside a shopping centre last night.

Police spokeswoman Susan Usher said the girl stabbed the 17-year-old boy about 8.15pm while they were on Cecil Avenue outside Carousel's Westfield Shopping Centre in Cannington.

"A 13-year-old was with a group of friends and it's alleged she became upset with a 17-year-old boy in the group," Ms Usher said.

"She removed a pocket knife from her trousers and stabbed him once to the left side of his back."

The boy was rushed to Royal Perth Hospital for treatment and is in a serious but stable condition.

The girl was charged with unlawful wounding and will appear in Perth Children's Court on September 6.