Najib's coalition suffered a shock defeat at the May 9 poll, beaten by a reformist alliance led by Mahathir Mohamad, which broke their six-decade stranglehold on power.
Mahathir, who first served as premier from 1981-2003 and came out of retirement aged 92 to take on Najib, and his allies focused on claims that the former leader and his cronies looted sovereign wealth fund 1MDB.
Billions of dollars were allegedly stolen from the fund in a sophisticated fraud, and used to buy everything from artworks to high-end real estate.
Najib and his reviled, luxury-loving wife Rosmah Mansor have had a swift fall from grace. They have been barred from leaving the country, and police have seized handbags, jewels and cash during raids on properties linked to the couple.
The ousted leader arrived at the Malaysian Anti-Corruption Commission (MACC) headquarters in the administrative capital of Putrajaya, walking through a scrum of about 100 journalists.
Najib was summoned by the MACC to give a statement over claims surrounding SRC International, an energy company that was originally a subsidiary of 1MDB, and forms just one part of the wide-ranging probe into the scandal.
SRC was placed under the finance ministry in 2012 -- Najib was finance minister as well as premier at the time.