"I could see bodies strewn to the left and right and in the middle of Las Ramblas was the body of a child, I knew what I had to do, I just raced to that child," he told Nine News.
"The first thing I tried to do was check his pulse to see if she was alive and he had no pulse and I kept my hand there.
"I tried to talk to him, I said a little prayer."
Police had ordered everyone, including Athwal, to leave Las Ramblas after the attack - but he refused to leave the boy.
But despite his bravery, Athwal insisted he's not a hero, but just a "common man, same as anybody else".
Speaking to Good Morning Britain, Athwal, a project manager, said he had known straight away that a terrorist attack had taken place.
He said: "You hear this screaming, thuds and thunder, you were just trying to think ''what I have I just seen?''
"Straight away I knew it was a terrorist attack. I didn't know what to do but I knew I had to do something."
Athwal explained that when he saw the little boy lying on the pavement, it reminded him of his son and he felt the need to rush to his side.
He added: "When I saw that child, I was instantly drawn to that child. He was quite badly injured, I checked for a pulse and he didn't have a pulse.
"I put a hand on his back and he wasn't breathing, so all I could do was sit with him and stroke his back until the police came."
