"Rosmah has been arrested," her lawyer K. Kumaraendran told AFP News Agency after she was questioned for hours at the Malaysian Anti-Corruption Commission (MACC) on Wednesday.
Investigations into the alleged looting of state fund 1MDB by an audacious international fraud ring have already seen her husband hit with more than two dozen charges including corruption and money laundering.
"Following this, Rosmah will face a number of charges," it said, adding she will be brought to court early on Thursday to be charged.
An MACC official said Rosmah could face up to 15 years in jail if convicted on each charge.
She is expected to spend the night at the MACC headquarters located in the administrative capital Putrajaya outside Kuala Lumpur.
Rosmah, widely reviled in Malaysia due to her profligate spending while her husband was in power, arrived at the MACC headquarters in a two-car convoy late Wednesday morning.
At the same time, her husband was also being questioned by police in a separate location in relation to the scandal.
Najib, 65, has denied any wrongdoing despite revelations that hundreds of millions of dollars ended up in his personal bank accounts.
Najib and Rosmah have been barred from leaving the country and enforcement agencies have relaunched a probe into how the 1MDB funds went missing.