A senior Malaysian journalist who quit his job at a leading newspaper said Prime Minister Najib Razak’s government has cracked down on freedom of speech as it tries to limit the fallout from a graft scandal surrounding a state investment fund.

Mustapha Kamil, the former group editor of the English-language New Straits Times, which is controlled by Mr. Najib’s ruling party, took a rare public stance by saying that an increasingly “authoritarian” stand by the government toward media was the reason he quit the newspaper in April....