A Taiwanese court found former president Ma Ying-jeou guilty in a political leaks case, his first conviction in a raft of lawsuits brought against him since he stepped down two years ago.
Ma was sentenced to four months in prison for violating the communication security and surveillance act, said the high court, overturning a previous not guilty verdict from a lower court.
The former president said Tuesday he will appeal the sentence and remains free.
Ma was also convicted of breaching the personal data protection act and of "using his presidential power not for executing presidential legal duties", the court added.
Under Taiwanese law any sentence under six months can be paid off with a fine.
The high court found that Ma had attempted to damage the "character and rights" of opposition lawmaker Ker Chien-ming when he leaked confidential information about an ongoing probe associated with Ker.
Investigators had been accused of tapping Ker's phone to acquire the information.
The information that he obtained through his office "should have been kept secret", a statement from the court said.
The leaks controversy sparked a political storm in 2013 and saw two top officials resign, while thousands of protesters took to the streets to demand Ma step down.
The DPP, then in opposition, compared the probe to the Watergate scandal in the United States.
The probe revolved around information gained from surveillance on Ker which implicated the then parliamentary speaker, a rival of Ma, in influence peddling.
Ma was not present when the verdict was announced. He had maintained his innocence throughout the trial and accused the prosecution of basing their case on flimsy evidence and speculation.
Ma's office said he "deeply regrets" the verdict and that it had failed to take into consideration "that a president needs to fulfil his commitment to the constitution and his duties to the country".
Last year, the ex-president was cleared of defamation charges and leaking confidential information by the Taipei district court in two separate cases also relating to the 2013 judicial probe into the parliamentary speaker.
While still in office Ma was protected by political immunity.
But since he stepped down as leader in May 2016 the 67-year-old has been hit with a string of corruption and other allegations.
Ma's Beijing-friendly Kuomintang (KMT) party held power from 2008 to 2016, before it was trounced by Tsai Ing-wen and her Democratic Progressive Party (DPP).
Ma was the third ex-president in Taiwan to be indicted on criminal charges.
His predecessor Chen Shui-bian was serving a 20-year sentence for corruption when he was freed on medical parole in 2015.
Lee Teng-hui was charged with embezzling state funds during his 1988-2000 presidency, but was acquitted.