The incident that occurred at the Sloane Square store, owned by John Lewis Partnership at around 1pm on December 30, with police saying they will investigate how the man fell.
The man is believed to have sustained life-threatening injuries and is in a critical condition. He is being treated for head and chest injuries at St Mary's Hospital, Paddington.
A spokesman for the Metropolitan Police said:
We were called at 1.05pm today to reports of a man fallen from an escalator inside a department store in Kensington and Chelsea.
Officers from the London Ambulance Service and the London Air Ambulance were also at the scene. A man, believed to be in his 20s, was taken to hospital with multiple injuries.
Those injuries are thought to be life-threatening at this stage. Officers remain at the scene and our enquiries continue.
In November, a man fell to his death from the third floor of Debenhams store in central London. The London Air Ambulance were also called in and treated five people for shock.