Monday's sentence, if upheld through appeals, could force Yoon's People Power Party to return 39.7 billion won ($A38.7 million) in election expenses reimbursed by the National Election Commission following his presidential election victory.
The Seoul Central District Court found Yoon made false statements during the 2022 presidential campaign when he denied introducing a lawyer to a former tax official and claimed he and his wife, former first lady Kim Keon Hee, had not met shaman Jeon Seong-bae.
Both relationships had come under scrutiny during the election campaign.
The court accepted prosecutors' arguments that Yoon had introduced the lawyer to the former official and maintained a long-standing relationship with Jeon after first meeting him through Kim.
"When a candidate personally announces false information at public events with a large impact, such as debates and interviews, the effect on voters' decision making is very significant," the court said in its ruling.
Prosecutors had sought a two-year prison term, arguing Yoon's remarks helped ease scrutiny of the allegations during the presidential race.
Yoon, who had denied any wrongdoing, plans to appeal the verdict, his legal counsel said in a statement.
Under South Korea's election law, political parties receive state reimbursement for campaign expenses if their presidential candidate wins or secures at least 15 per cent of the vote.
If a candidate later receives a final election-invalidating sentence, such as a jail term or a fine of at least 1 million won, the party must return the funds.
Yoon is also facing other criminal trials, including an insurrection case related to his December 2024 martial law declaration.
In February, a lower court sentenced him to life imprisonment over the case.
