Tatsuya Kato, who until October 1 was bureau chief of Japan's conservative Sankei Shimbun newspaper, is charged with criminal libel punishable by up to seven years' jail.
The charge stems from an August article he wrote about Park's whereabouts on the day the Sewol passenger ferry sank in April with the loss of 300 lives.
At a preliminary court hearing last month the journalist insisted he had acted in the public interest and had no intention of defaming Park.
"I was indignant at his article, which defamed our president with groundless allegations," Chang Ki-Jung told the court.
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Kato's article picked up rumours circulating in the South Korean media and brokerages that the unmarried Park had disappeared for a tryst with her former aide Jeong Yun-Hoe at the time of the sinking.
Park has condemned "groundless" allegations over her personal life and administration.
Chang told the Seoul Central District Court that Kato had aggravated ties between Tokyo and Seoul by picking up unconfirmed rumours.
The court accepted a request from Kato's lawyers to summon Jeong as a witness on January 19.
South Korean defamation law focuses on whether what was said or written was in the public interest -- rather than whether it was true.
Media freedom group Reporters Without Borders has criticised the trial, arguing that Kato's report was clearly in the public interest.
The trial comes at a time of a widening political scandal over allegations by the Segye Times newspaper last month that Jeong meddled in state affairs.
Park's office insisted the document was inaccurate. Prosecutors have questioned Jeong and presidential officials over how the document was leaked.
The president's younger brother, Park Ji-Man, was questioned Monday over allegations that he also received leaked documents from presidential officials.