The US President Donald Trump has increased tariffs on US$200bn (€178bn) in Chinese imports - heightening tensions with Beijing.
The tariff hike came despite American and Chinese negotiators meeting for last-ditch talks in Washington on Thursday.
Import taxes on products from 5,700 categories rose from 10% to 25% at 12.01am Washington time (5.01am Irish time).
Internet modems, furniture, lighting products, car parts, vacuum cleaners and building materials are all affected.
Some business leaders have warned that the escalation in the US-China trade war will hit consumers and small companies the hardest.
Gary Shapiro is chief executive of the Consumer Technology Association.
He said: "Our industry supports more than 18 million US jobs but raising tariffs will be disastrous.
"The tariffs already in place have cost the American technology sector about $1bn more a month since October.
"That can be life and death for small businesses and start-ups that can't absorb the added costs."
Economists say it could take three or four months before American shoppers begin to feel the pinch.
China had threatened to retaliate if the US president went ahead with the tariff hikes.
Negotiations aiming to break the current impasse are expected to continue on Friday morning, the White House said.
In a statement on Friday, China's Ministry of Commerce said that the government "deeply regrets that it will have to take necessary countermeasures."
"It is hoped that the US and Chinese sides will meet each other halfway and work together" to resolve their dispute, the statement added.
It did not specify what those countermeasures might be.