The verdict on Tuesday was the first from the bombings that killed 191 people and wounded 1900.
The juvenile - identified only by his initials GM - made a short appearance in the armoured basement courtroom in Madrid's High Court building.
Most of the 30 train bombing suspects under arrest or court supervision are North Africans described by High Court Judge Juan del Olmo as waging an Islamic holy war against the West.
In videotaped messages the bombers said they represented al-Qaida in Europe and said they were attacking Spain for sending troops to Iraq and Afghanistan.
The juvenile was a Spaniard from the northern region of Asturias who came into the plot through links to an older Spanish man who dealt in drugs and black-market explosives, prosecutors said.
In February, while others were stealing dynamite from an Asturias mine, he waited in a car outside. Moroccan suspects then took the explosives to Madrid in a car, he said.
He was later sent to Madrid on a bus to retrieve the car - but on the way he carried with him a backpack containing more explosives, for which he received $1550. Other explosives were paid for in Moroccan hashish.
The attack involved the use of
14 bombs; ten of them exploded
Police arrested two people and seized weapons, explosives, detonators and drugs on Tuesday in the same Asturias region where the explosives used in the 11 March attacks originated. It was also the same brand of dynamite.
In court, Judge Jose Maria Vazquez Honrubia asked the defendant if he agreed with those charges and the six-year term proposed by the prosecutor. He replied simply: "Yes."
The judge then sentenced him to six years in a juvenile detention centre followed by five more years of supervision.
The case came to trial a relatively swift eight months after the attacks, because the defendant is a minor. His associates called him El Guaje, meaning the Little Guy.
The stolen TNT was packed into 14 bombs of roughly 10 to 12kg each that were hidden in sports bags and left aboard four packed commuter trains.
Ten of the bombs went off at roughly the same time, three duds were destroyed by police and one - found 12 hours later in a police warehouse where it had been moved unintentionally - provided virtually all the breaks in the case, starting with the mobile phone used to trigger the detonator.