Near the start of the hearing, Yoon said he had worked in public service with "a firm commitment to free democracy", when invited by the acting chief justice, Moon Hyung-bae, to speak.
Dressed in a navy-coloured suit with a burgundy tie, Yoon, a career prosecutor before his 2022 election as president, pledged to answer any questions the court might have.
Judges will decide whether to remove Yoon from office permanently or reinstate him. (AP PHOTO)
Yoon has been incarcerated since last week under a separate criminal investigation into whether he led an insurrection by seeking to impose martial law in early December, which shocked the nation and was overturned within hours by parliament.
Before the hearing, his team of lawyers said in a statement Yoon intended to elaborate on his justification for declaring martial law on December 3 and request through his lawyers a list of witnesses he wished to be called.
The Constitutional Court began the trial on December 27 to review an impeachment motion that accused Yoon of violating his constitutional duty.
The judges will decide whether to remove him from office permanently or reinstate him.
Yoon may be questioned by Constitutional Court justices, a court spokesperson said.
Yoon's decision to attend the impeachment hearing contrasts with his vigorous resistance to criminal proceedings against him where he has refused to answer summons by investigators or attend interrogation sessions.
Yoon's legal team has denied he masterminded an insurrection, a crime in South Korea punishable by life imprisonment or even technically by the death penalty.
When oral arguments at the impeachment hearing began last week, lawyers for Yoon said the impeachment was a political attack against the president by opposition parties abusing their parliamentary majority and it had nothing to do with safeguarding constitutional order.
The main opposition Democratic Party, joined by minority parties and also 12 members of Yoon's People Power Party, voted with a two-thirds majority to impeach Yoon on December 14.
Security has been heightened at the Constitutional Court in central Seoul, after a mob of angry Yoon supporters went on a rampage through the district court that issued a warrant to extend his detention early on Sunday.
Dozens of police buses were lined bumper-to-bumper on both sides of the street in front of the court to limit access to the premises hours before the start of Tuesday's hearing.
Yoon was driven from the Seoul Detention Centre, where he is being held, in a correctional service vehicle escorted by a Presidential Security Service motorcade.
