The boy was arrested in west London this morning on suspicion of assault, police said. He is being held in custody.
The victim, a 20-year-old man, was stabbed towards the end of the Notting Hill carnival on Monday. He was taken to hospital where his condition was described as serious but stable after he was found with stab wounds to the stomach.
Today’s arrest is the fifth over the stabbing incident. The others arrested are being held at various London police stations.
A record number of 6,500 police patrolled the streets on Monday. Scotland Yard said the latest figure for the total number of arrests during the carnival weekend was 245, up slightly on a total of 243 last year.