Kadyrov posted the comments on Telegram, accompanied by a clip in which a young man in khaki clothing is seen punching and kicking another man cowering in a chair before wrestling him to the floor and slapping him on the head.
The Chechen leader, who styles himself as a ruthless ally of President Vladimir Putin, said he was releasing the video to settle any doubts about whether the incident, first reported last month, had really taken place.
"He beat him, and he did the right thing," Kadyrov said.
"Without exaggeration, yes, I am proud of Adam's action," he said, adding that he respected the boy for acquiring "adult ideals of honour, dignity and defence of his religion."
The prisoner, Nikita Zhuravel, had complained about the attack to Russia's human rights ombudswoman, who said last month she had referred the issue to her counterpart in Chechnya.
Kadyrov, 46, has ruled Chechnya with an iron hand since becoming president in 2007, following in the footsteps of his father Akhmat who was killed in a bomb blast in 2004. He has enjoyed generous funding from Putin to rebuild the mainly Muslim region which suffered devastation in two wars following the collapse of the Soviet Union when it tried to break away from Moscow's control.
Kadyrov has been giving increasing publicity to his three teenage sons, whom he said last year he was sending off to fight for Russia in the Ukraine war. The extent of their actual involvement in any combat action is not clear.
The eldest, Akhmat, was pictured with Putin in the Kremlin in March, fuelling rumours he was being groomed as Kadyrov's successor.
Kadyrov's health has been the subject of intense speculation, with rumours swirling this month that he was dead or in a coma. Last week he published a post on Telegram saying that he was fine, and that his reason for travelling to a Moscow hospital was to visit the bedside of a sick uncle.
(Reporting by Mark Trevelyan; Editing by David Gregorio)
By Mark Trevelyan
