He had pleaded not guilty to the charges of criminal breach of trust, money laundering, and abuse of power.
The case against Najib, in office from 2009 to 2018, was widely seen as a test of Malaysia's anti-corruption efforts.
The scandal around Malaysia's 1MDB sovereign wealth fund has uncovered a global web of fraud and corruption.
"After considering all evidence in this trial, I find that the prosecution has successfully proven its case beyond a reasonable doubt," judge Mohamad Nazlan Mohamad Ghazali told the Kuala Lumpur High Court.
Tuesday's verdicts centred on 42 million ringgit ($10m, £7.7m) transferred from the fund to the then-prime minister's private accounts.
Najib denies all wrongdoing and says he was misled by financial advisers - in particular fugitive financier Jho Low.