Loui Karl Phillips was killed in the village of Monk Bretton, near Barnsley, on August 8 as he cycled along Fish Bank Lane with a friend.
A 17-year-old who cannot be named for legal reasons approached him and stabbed him once in the chest before calmly walking away.
Although the two boys had not met, the attacker was aware of Loui's identity and was said by police to be 'jealous' of him.
He appeared at Sheffield Crown Court on Monday and admitted the attack, which he had previously denied.
Loui was rushed to hospital where he underwent open heart surgery in A&E to remove the blade. He was then transferred to the specialist cardiac unit where he underwent further surgery. Loui died on August 12.
The Barnsley FC fan's father died in 2020 and he left behind a younger brother. The keen footballer played on the wing for Pinfold Pumas, where he was known for his speed.
Detective Chief Inspector Mark Oughton from South Yorkshire Police said: “This is an incredibly tragic case where an innocent young boy was stabbed to death because his attacker was jealous.
“The killer planned his attack, locating Loui and his friend having purposely set out to go and stab him. He has shown no remorse for his actions and always maintained his innocence until Monday.
“Loui was still conscious after he was stabbed and managed to give a full account of the attack to a police officer on the way to the hospital. All he wanted to know was where his mum was.
“My thoughts are with Loui’s mum, and his family and friends today; they have suffered unimaginable loss this year and no court result can ever make up for that.”
The teenager will return to Sheffield Crown Court for sentencing on December 17.