The opinion poll conducted by Merdeka Centre among 1,055 registered voters in December showed Mr Najib's approval rating has declined to 52% from 62% in August when the centre's last poll was conducted.
His popularity appears to have been dented by the government's reduction of fuel subsidies in September, its plan to raise the electricity tariff by 15% from next month and the planned introduction of a 6% goods and services tax next year.
"Concern over the economy, particularly in the form of rising costs and inflation, rose to its highest ever recorded level of up to 67% of all respondents," the independent centre said in a statement.
In Malaysia's race-based politics, the centre said, the slide in Mr Najib's approval ratings cut across all races although support for him among the dominant ethnic Malays remained strongest at 66% compared with ethnic Chinese at 21% and ethnic Indians at 57%.
Ethnic Malays, comprising over 60% of the country's 29 million people, form the core support of Mr Najib's party, the United Malays National Organisation.
Mr Najib has recently vowed to expand the privileges accorded to the Malays, perceived by his government as economically lagging behind the minority Chinese.