Jack Dudley came to the rescue of Sam, three, when he saw him sinking to the bottom of an indoor pool at home.
He said he was inspired by the hero of the TV cartoon Ben 10. "I am happy Sam is all right because he is my baby brother and we are good friends," he said.
"I watch programmes like Ben 10 which have heroes and I'm glad I saved him. I saw him jump inb and he was going under in the deep end. I tried to grab him but It was hard because he is quiote heavy.
"It was hard to keep his head above water but we got him out. My mum was shouting and my dad started pumping water out of him. I thought he was dead."
Jack, of Brayton, near Selby, who has taken swimming lessons for three years, saw Sam take off his supportive armbands before jumping in to the pool.
"He made a jump for the inflatable," said the boys' father Pete Dudley, 45, who is commercial manager for York City Knights RLFC.
"But he missed and started sinking in the bottom. Jack tried to pull him up but he slipped from his grasp. So he dived under again and this time managed to get his brother to the surface.
"He got his hand under Sam's chin, kept his head out of the water and swam to the side pulling his brother along."
Mrs Kim Dudley, 41, helped lift Sam from the pool and called an ambulance while her husband tried to resuscitate him.
He was unconscious and lifelless but Mr Dudley compressed his lungs and he began to gurgle and cry. Sam was allowed home after an overnight stay in hospital to check for water in his lungs.
"We are so proud of Jack," added Mr Dudley. "To show that level of composure is incredible."
The two boys had been out of view of their parents for 30 seconds when the incident happened.