The country's immigration department said in a brief statement on Saturday that Najib and his wife, Rosmah Mansor, have "just been blacklisted from leaving the country".
"I have been informed that the Malaysian Immigration Department does not allow me and my family to go overseas". "I respect the directive and will remain with my family in the country".
Earlier this week, Mr Najib's long-ruling Barisan Nasional coalition suffered a shock electoral defeat.
Mahathir said there was sufficient evidence for an investigation into Najib, in reference to a massive corruption scandal involving the 1MDB state fund that Najib set up and is being investigated in the United States and other countries.More news: After "crazy hard" development, SpaceX's Block 5 rocket has taken flight
Speaking to the media at the time and in response to a journalist's question, then-former Malaysian PM Najib Razak said that the outcome of the Malaysian general election (GE14) would not change the nature of relations between Malaysia and Singapore.
Malaysian Prime Minister Mahathir Mohamad said on Friday (May 11) the country's monarch had indicated he was willing to grant a full pardon to jailed politician Anwar Ibrahim.
Pakatan Harapan (PH) will have a small Cabinet line-up to start with, with 10 key ministerial positions to be filled, Prime Minister Tun Dr Mahathir Mohamad said yesterday.
Now the world's oldest elected leader, Mahathir, 92, was sworn into office by Malaysia's constitutional monarch late on Thursday. As a white van with heavily tinted windows attempted to pass by, the crowd surrounded the van and demanded it be opened so they could see who was inside. The ex-leader's wife is hugely unpopular, due to her imperious manner and a reported love of luxury shopping trips.
Najib said his deputy Ahmad Zahid Hamidi, who is also the former deputy prime minister, would take over as the new president of UMNO, the main component of the alliance.More news: Xiaomi's smart home products now work with the Google Assistant
Filings by the US Justice Department in a civil lawsuit indicated almost $30 million of the money stolen was used to buy jewellery for Rosmah, including a rare 22-carat pink diamond set in a necklace.
Also Saturday, Mahathir appointed Lim Guan Eng, an ethnic Chinese, as finance minister, the first ethnic minority to be appointed to the powerful post in 44 years. Mahathir has said he plans to eventually hand the premiership to Anwar.
Anwar was a senior figure in government and Mahathir's heir-apparent until the pair dramatically fell out.
Anwar was jailed again during Najib's rule. Anwar's wife, Wan Azizah Wan Ismail, endorsed Mahathir and served as his deputy in the coalition to oust Najib.More news: Awesome timelapse footage shows Hawaii lava flow devouring auto