Police are investigating after the incident in Ribbleton Lane, Preston on Saturday night which caused the road to be close for a time and traffic diverted.
Officers believe it was a targeted attack and say the boy's injuries "do not appear to be life-threatening". No arrests have yet been made.
A Lancashire Police spokesman said: "We were called at around 8.30pm on Saturday, December 4 to reports of an assault on a teenage boy on Ribbleton Lane, Preston.
"We attended and found a 13-year-old with stab wounds to his leg. He was taken to hospital and thankfully at this time his injuries do not appear to be life-threatening.
"No arrests have been made as yet but an investigation has been launched and anybody with information is asked to contact us straight away.
Det Ch Insp Mike Gladwin, of Preston Police, said: “This is a shocking attack which has left a teenage boy in hospital with stab wounds.
"Our inquiries are very much underway and we would ask anybody with information about it to get in touch with us as soon as possible.
“At this time we believe it was a targeted attack and there is no wider threat to the public, however patrols will be stepped up in the area to offer reassurance to residents.”
Anybody with information should contact us on 101, quoting log 289 of December 4.