A video filmed at Chiba Zoological Park shows the toddler and predator locking eyes for a few seconds before the boy turns his back. Instinct takes over and the lion rushes at him and pounces, only to slam into the glass wall of his enclosure.
The child is seen jumping backwards in fright.
While it might appear that the lion showed signs of hunting its prey, zookeepers said the 181kg cat simply wanted to play.
However, lion expert Adam M. Roberts, who is CEO of Born Free USA, told The Dodo: "Lions are natural wild predators and the child in this video, especially when turning his back to the massive feline, becomes prey in the animal's eyes. But luckily for the family, if the barrier had not held the consequences could have been catastrophic."
Baby zoo animals around the world
Lion charges at toddler at zoo, smashes into glass wall
These babies, named Sri Kandi (left) and Arjuna (right) were born at Tierpark, Berlin, Germany. They made their debut on 5 March 2012.
Clinging to his mum, baby Changi meets his public for the first time at Krefeld Zoo, Germany.
This little rhino, named Male, was born in January. He's pictured here with his mother, Mana, at Magdeburg Zoo, Germany.
The zoo at Tierpark in Berlin celebrated the birth of a litter of four dingo babies in January.
Only a few days old, this baby giraffe, named Kimarle, peaks out from her enclosure at the ZOOM Zoo in Gelsenkirchen, western Germany. The 170cm tall giraffe was born on March 9.
A one-month-old baby squirrel monkey clings to its mother's back at Qingdao Forest Wildlife Park, China.
A two-week old baby gorilla is cuddled by her mum, Mumba, a Western Lowland Gorilla, at Port Lympne Wild Animal Park near Ashford, Kent.
Born at Cincinnati Zoo, US, this one-month old hedgehog is being hand reared by keepers until it is old enough to be included in the zoo's outreach programme.
Ludwig the baby elephant is clearly happy to be out and about at the Tierpark Hellabrunn Zoo, Munich.
These cuties were born in February at Bristol Zoo Gardens.
This cub was born prematurely at Toronto Zoo. His mum rejected him soon after he was born, so he was cared for by staff at the zoo. Here he is making his debut in February.
This six-month old baby Western Lowland Gorilla is seen here enjoying the spring sunshine with her mother at Dublin Zoo.