The woman, in her 20s, suffered head injuries following the fall from the 11ft (3.3m) sculpture.
The London Air Ambulance was called on Saturday evening and she was treated for 30 minutes before being taken to hospital.
Her injuries are no longer life-threatening and police are not treating the incident as suspicious.
In past years, the Greater London Authority has called for people to be banned from climbing on the lion statues, to protect them from being damaged.
It said tourists had damaged the 148-year-old lions and this damage would worsen without intervention.
But English Heritage is against a ban and said it would prove "deeply unpopular".
Trafalgar Square lion facts
A woman was hurt when she fell from one of the bronze lion sculptures in Trafalgar Square, police said.