The rights group said it interviewed 23 boys aged between 14 and 17 who were charged with, or convicted of, belonging to IS, and that 16 of them said they had been "tortured" during questioning.
Some boys said members of the Kurdish security forces known as Asayesh beat them with plastic pipes, electric cables or rods while others said they were subjected to electric shocks or a painful stress position dubbed the "scorpion", the watchdog said.
"Several boys said the torture continued over consecutive days, and only ended when they confessed" to involvement with IS, it said.
"Most said they had no access to a lawyer and they were not allowed to read the confessions Asayesh wrote and forced them to sign," it added.
It said the punishment inflicted by security forces of the Kurdistan Regional Government in northern Iraq took place in 2017 and 2018 despite promises by authorities to investigate the torture claims.
"Nearly two years after the Kurdistan Regional Government promised to investigate the torture of child detainees, it is still occurring with alarming frequency," said Jo Becker, children's rights advocacy director at HRW.
The watchdog said its staff interviewed the boys during a November visit to a detention centre in Arbil, where 63 children are being held.
A senior Kurdish official dismissed the allegations.
Dindar Zebari, international affairs adviser to the Kurdish government, told AFP that "HRW never visited" the detention centre.
"No one can be arrested unless the judiciary authorises it. And any person who has been arrested is treated in accordance with the law," he said, adding that the Kurdish government rejects the use of torture.
According to HRW, most of the boys said their interrogators told them what they should confess and many said they gave false testimony only to stop the torture.
"My confession says that I joined ISIS for 16 days, but actually I didn't join at all," a 16-year-old told HRW using another acronym for the jihadist group.
A 14-year-old said: "First they said I should say I was with ISIS, so I agreed. Then they told me I had to say I worked for ISIS for three months. I told them I was not part of ISIS, but they said, 'No, you have to say it'."
The boy said after two hours of interrogation and torture he agreed to their demands.
"The Kurdistan authorities should immediately end all torture of child detainees and investigate those responsible," HRW said.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)