Kilicdaroglu, head of the secular Republican People's Party (CHP), visited journalist-turned lawmaker Enis Berberoglu in his prison in the Istanbul district of Maltepe, a party spokesman told AFP.
Berberoglu was arrested last month for leaking classified information to an opposition newspaper and Kilicdaroglu immediately began a foot march from Ankara to Istanbul to protest his plight.
The march culminated in a rally on Sunday that attracted hundreds of thousands of people, the biggest event that critics of President Recep Tayyip Erdogan have staged in years.
"Berberoglu found the march 'impressive' and thanked the party leader," the CHP spokesman said. "He is writing a book," he added, without elaborating on its content.
Kilicdaroglu walked 450-kilometres (280 miles) in 25-days, with the march becoming a rallying point for all those protesting against alleged injustices under Erdogan.
About 50,000 people have been arrested under Turkey's state of emergency, imposed after last July's failed coup, and another 100,000 have lost their jobs, including teachers, judges, soldiers and police officers.
"People who have committed no sin, who have absolutely no link to terror, people who are wrongly accused are being put in jail, hurting the conscience of society," Kilicdaroglu was quoted as saying by Turkish media after the meeting with Berberoglu.
"Enis is one of these people," he added.
The government dismissed the march as a stunt but the police did not impede its progress and also provided security for the final rally. Erdogan has so far not commented on the magnitude of Sunday's giant meeting.
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