Cameron Comey had been playing with his brother by the swollen River Towy in Carmarthen on 17 February, 2015.
The inquest in Milford Haven heard Cameron's brother Anthony ran home and said the boy had fallen into the river at the bend known locally as "the 22".
Coroner Gareth Lewis concluded the death was accidental.
He added it was a "truly awful case".
Mr Lewis told the hearing he had been authorised by the chief coroner to proceed with the inquest in the absence of the body.
A full scale search involving fire, coastguard, search and rescue and dogs was launched after the incident and continued for several months, but Cameron's body has never been found.
Pembrokeshire Coroner's Court heard Queen Elizabeth High School pupil Cameron had earlier been out shopping to Carmarthen with his mother Amanda, his brother and baby sister before returning home.
Coroner's officer Malcolm Thompson said the youngster had asked his mum if he and younger sibling Anthony could go out and play - which she agreed as long as he had his mobile phone with him.
The hearing was told that at around 14:45 GMT an unnamed businessman saw the two brothers playing near the riverbank.
Mr Thompson added: "The brothers must have seen the man because he saw them trying to hide behind a pile of logs.
"He returned to the area (around 10 minutes later) but could not see the two brothers."
Then at around 3.30pm, nine-year-old Anthony ran back home - arriving out of breath and covered in mud - saying Cameron had fallen into the river.
Neither of Cameron's parents were at the short hearing into their son's death on Thursday.
The danger of playing near rivers has been highlighted by the death of an 11-year-old boy whose body has never been found, a coroner has said.