Tatsuya Kato was put on trial in December last year on a charge of criminal libel for an article he wrote questioning where Park was on the day the Sewol passenger ferry sank, killing 300 people.
The article picked up unproven rumours circulating in the South Korean media that the unmarried Park had disappeared for a tryst with her former aide at the time of the disaster.
A Seoul court said today Kato's article had damaged Park's reputation as an individual, but accepted his argument that it was intended to serve public interest, finding him not guilty of the charge.
It added the freedom of press "must be respected to the full" for the development of democracy.
Seoul prosecutors had demanded a guilty verdict and an 18-month jail term for Kato, arguing he clearly intended to defame Park.
However, the court cleared him.
