"The decision to undo all of this, to block all of it, seems wrong to me," Zelenskiy wrote on X.
"It definitely helps only the Russians."
An executive order issued by Trump on Friday night waived key Russia-related US sanctions and tariffs - including duties on the country's goods and penalties for buying its oil and gas - that had been scheduled to start next week under the legislation known as the Graham Act that he signed into law last month.
Trump said the waiver was in the national interests of the United States.
The law is meant to increase pressure on Russia for its invasion of Ukraine that began in 2022.
Trump, however, has been warming toward Moscow while publicly criticising Zelenskiy, including suggesting Kyiv needs a new leader.
Zelenskiy said Trump's latest action emboldens Russian President Vladimir Putin.
"It only fuels their satisfaction and their desire to keep killing people," wrote Zelenskiy, who said Russia struck more than 15 Ukrainian power generation and transmission sites on Friday.
"This certainly does not stop the war."
The Ukrainian leader accused Russia of planning to ramp up strikes in western Ukraine.
"Is Russia preparing for this? I think so," Zelenskiy wrote.
Russia's investment envoy, Kirill Dmitriev, called the Trump waiver "a truly historic moment", posting on X on Friday that had the penalties taken effect, they could have effectively brought all economic co-operation between Russia and the US to a halt.
Weeks before US midterm elections in which Trump's Republicans are battling to maintain their narrow grip on both chambers of Congress, the new measures could have risked further raising oil prices as Americans face increased costs largely attributed to the US-Israeli war with Iran, analysts have said.
They would have imposed tariffs of up to 100 per cent within 30 days on all goods brought into the US from the five largest importers of Russian crude oil or gas, as well as any country that knowingly bought oil or gas 30 days after the law was enacted or was among the top five countries helping Russia evade sanctions.
China and India, which are among the biggest buyers of Russian oil, however, had raised concerns about the Graham Act.
Beijing has said it "consistently opposed long-arm jurisdiction that lacks a basis in international law" while India warned Washington that new tariffs could impact bilateral ties.
Friday's executive order said the waiver would remain in effect until Trump or his top officials determine that "further action is appropriate".
Meanwhile, Ukraine attacked targets in St Petersburg with dozens of drones, including an oil terminal at the port and a data centre, according to Russian reports.
The military shot down 126 Ukrainian drones in the region, news agency TASS reported, citing the governor of the province of Leningrad that surrounds the city, Aleksandr Drozdenko. No casualties were reported.
The extent of the refinery damage remained unclear. Unverified videos circulating online appeared to show a large fire.
The data centre that was struck belongs to the internet company VK, which operates a social network popular in Russia, according to TASS.
St Petersburg airport temporarily suspended operations because of the Ukrainian attack with passengers and staff moved to safety.
