Jonty Bravery, 18, has pleaded guilty at the Old Bailey to attempted murder after throwing a six-year-old boy from the tenth-storey viewing platform of the Tate Modern in August.
Bravery, who lived in Ealing, but was originally from west London, had been expected to go on trial in February.
But he admitted the charge during a short hearing at the Old Bailey on Friday.
He was remanded in custody by the judge, Recorder of London Nicholas Hilliard QC, ahead of his sentencing hearing at a later date.
Bravery was arrested after approaching the youngster at random while at the Tate and then throwing him from the viewing platform.
The victim had been visiting the Tate Modern gallery with his family who were on holiday in London.
He suffered a deep bleed on the brain as well as fractures to his back, arms and legs.
Bravery was apprehended by members of the public, who witnessed the incident and held until police arrived.
The boy, who has not been named, was initially in a life threatening condition, but since then has made steady progress and while he can still not speak, his family recently said he was doing amazingly well.