Aged 74, Mr Lai, one of Hong Kong's best-known activists, is behind bars for his role in pro-democracy protests that rocked the Chinese territory three years ago.
Along with other executives of Apple Daily, a newspaper critical of Chinese power that was forced to close, Mr. Lai will be tried in December for "collusion with foreign forces".
If found guilty, he faces life in prison under the national security law imposed in 2020 by Beijing on Hong Kong to end all dissent.
His conviction for fraud on Tuesday is the fifth that the Hong Kong prosecution has obtained against Mr. Lai in 18 months.
The defendant and two other former executives of Apple Daily, Wong Wai-keung and Chow Tat-kuen, were being prosecuted for having breached the terms of a rental agreement, signed with the company Hong Kong Science and Technology Park (HKSTP), founded by the government to provide office space.
Prosecutors said Mr. Lai housed a consulting firm he operated for personal use in the daily's building, which was leased to the HKSTP for the specific purpose of publishing and printing.
One of the three defendants, Mr Chow, testified for the prosecution in exchange for his acquittal.
Judge Stanley Chan found Mr. Lai guilty of two counts of fraud, an offense which carries him up to 14 years in prison.
His co-defendant, Mr. Wong, was convicted on one count of fraud.
Their sentences will be announced at a later date.
© 2022 AFP