The 17-year-old, who cannot be named for legal reasons, appeared at Sheffield Crown Court on Monday and admitted stabbing and killing 15-year-old Loui Phillips on August 8.
The youngster was riding his bike with a friend along Fish Dam Lane, Carlton, at around 2pm when the killer approached him from the opposite direction.
Loui, who had never met his attacker before, was stabbed in the chest.
He was then rushed to hospital where he underwent open heart surgery in A and E to remove the blade, before he was transferred to the specialist cardiac unit where he underwent further surgery.
Loui died on August 12 despite the best efforts of the emergency services.
The senior investigation officer in the case, DCI Mark Oughton, 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.”
The 17-year-old will return to Sheffield Crown Court on December 17 for sentencing.