The defendant, an Austrian identified as Beran A, was arrested on August 7, 2024, the day before the first of three planned concerts by the US pop star in Vienna.
All three dates were then cancelled, to the dismay of fans and of Swift herself, who wrote afterwards that it was "devastating".
The trial in Wiener Neustadt, near Vienna, is focused on more than that planned attack.
Beran A is also accused, along with Slovak national Arda K, of planning attacks in the Middle East they did not go through with, and of providing moral support to a third man who has been arrested on suspicion of carrying out a knife attack in Mecca.
"I plead guilty in part," Beran A said at the start of his questioning by the presiding judge.
Asked if he pleaded guilty to the charges relating to the planned concert attack, he said: "Yes."
Both defendants covered their faces as they entered the courtroom, as is common in German-speaking countries to avoid being identifiable in photographs.
Beran A wore a dark blue shirt and jeans, Arda K a light blue shirt and beige trousers.
Beran A, who prosecutors say swore allegiance to Islamic State, was accused of using video instructions by Islamic State on how to make a shrapnel bomb, producing a small amount of the explosive triacetone peroxide and illegally trying to buy weapons including a machine gun and hand grenade for the planned attack.
He has been charged with various terrorism-related offences as well as belonging to a criminal organisation and making a dangerous threat, and faces 10 to 20 years in prison if convicted.
Prosecutors also allege the two and the third man, all school friends, had planned to carry out one attack each in the Middle East before the Swift concerts, in March 2024: Beran A in Dubai, Arda K in Istanbul and the third man in Mecca.
While each travelled to his designated city, only the third man is believed to have launched an attack - he was arrested on suspicion of stabbing a security official at Mecca's Grand Mosque, and remains in custody in Saudi Arabia.
Arda K pleaded guilty to travelling to Istanbul with the intention of carrying out a militant attack, as did Beran A in relation to his trip to Dubai.
They pleaded not guilty, however, to providing moral support to the third man, arguing he was already determined and the driving force behind those plans.
Tuesday is the first of four scheduled trial days, with the last on May 21.
15°C