Kushner and Witkoff travelled from Moscow, where they held three hours of talks with Russian President Vladimir Putin on Saturday, but did not appear to secure a breakthrough in the conflict.
Previous rounds of US-mediated peace talks encouraged by President Donald Trump, who campaigned on a promise to quickly end the war, have yielded few results.
He suggested on Friday the US had a fresh proposal, but did not elaborate.
Putin described Saturday's meeting as "highly useful", while a White House official told Reuters next steps would be announced in the coming weeks.
The two envoys met Zelenskiy and other top Ukrainian officials in central Kyiv on Sunday.
Zelenskiy, who believes Washington will be key to any peace deal, has said a window of opportunity to make peace exists before the US begins focusing on its next presidential elections.
In a post on social media before the envoys' arrival, Zelenskiy said he had held a preparatory meeting with his negotiating team and that Ukraine wanted an end to the war.
"Security guarantees are needed. A dignified post-war peace is needed. Our proposals have been drawn up," he said.
A source familiar with the matter told Reuters the British, French and German national security advisers were also in Kyiv on Sunday.
The source did not specify which meetings they would participate in.
Witkoff and Kushner, Trump's son-in-law, have visited Moscow several times in their roles as peace envoys.
But Sunday marks their first visit to Kyiv in their capacity as US negotiators.
Their visit has long been sought by Ukraine's leadership. On the streets of Kyiv, which has been bombarded heavily in recent weeks, some residents were hopeful.
"The issue can be solved in the blink of an eye. We want peace," military veteran Stepan Yermak said.
"I've fought myself, so I know what war is like. We need peace, because there's no other way."
In previous talks, the sides have remained far apart on a range of key issues, including the future of the eastern Ukrainian region of Donbas.
Russia has demanded Ukraine's withdrawal from an entrenched belt of towns that it still controls there, but Kyiv has refused, reasoning it could hold on there for years.
For Ukraine, hard security guarantees from its European allies and the US are crucial to deterring Russia from invading again.
"We need an agreement that will never be broken," said Svitlana Kurylenko, a 19-year-old cafe worker.
"Just leave Ukraine alone and let people live in peace."
Though the front lines of the ground war remain largely static, both Russian and Ukrainian forces have intensified long-range air strikes in recent weeks.
Russia has pounded the Ukrainian capital day and night with missiles and drones for the past two weeks, including an attack on Friday on the headquarters of the SBU security service.
The Kremlin has said it would pause strikes on Kyiv until Monday, while Zelenskiy has said Ukraine would also halt strikes on Moscow.
There was no let-up in fighting along the 1200km front, however, and Russia bombarded other Ukrainian cities on Saturday.
Ukraine has inflicted damaging hits throughout the summer on Russian refineries, oil terminals and the warehouses of online retailer Wildberries, in an attempt to raise the cost to Moscow of continuing the war.
Ukraine said on Sunday it had struck the Ryazan oil refinery, about 480km inside Russia, overnight, causing a fire.
