China will stop importing North Korean iron ore, coal, fish and other goods on September 5 under UN sanctions imposed over the North's nuclear programme, officials in Beijing said.
China, the isolated North's main trading partner, has been reluctant to push leader Kim Jong Un's regime too hard amid fears it might collapse.
However, Beijing is increasingly frustrated with Pyongyang and joined in approving a UN Security Council ban on August 5 on coal and other key goods.
The Chinese customs agency's announcement follows an escalating exchange of angry words between US president Donald Trump and Mr Kim's government.
Mr Trump declared that the US military was "locked and loaded", while Pyongyang threatened to fire four missiles into waters near the US territory of Guam.
The latest UN sanctions are intended to block North Korean exports worth one billion dollars (£768 million) - a significant share of total exports valued at three billion dollars (£2.3 billion) last year.
China stopped importing North Korean coal in February but total trade has risen, which prompted Mr Trump in July to accuse Beijing of failing to use its economic leverage to stop Pyongyang's pursuit of nuclear weapons.
Beijing had long been Pyongyang's only diplomatic defender but agreed in March 2016 to sanctions against the North following its test of a long-range missile.
Chinese leaders still oppose blocking all trade with the impoverished North, which they argue might cause widespread hunger.
The Chinese import cut will come the week of Foundation Day on September 9, the North Korean holiday commemorating the founding of the country's government in 1948.
North Korean imports of Chinese oil, consumer goods and other products rose 36.7% from a year earlier in the first half of this year to 1.1 billion yuan (£126 million), according to Chinese customs data.
Chinese imports from the North fell 7.6% in the same period to 611 million yuan (£69 million), squeezing the North's revenue.