Zelenskiy and Witkoff clasped one another in a hug as they went into the talks hosted by the leaders of Norway and Finland in Oslo.
Trump's son-in-law Jared Kushner was attending remotely.
Zelenskiy met Norway's King Haakon VIII before talks with Norwegian Prime Minister Jonas Gahr Stoere and Finnish President Alexander Stubb ahead of his meeting with the US delegation.
Ukraine says the US has lately failed to provide early warning of Russian missile launches, throttled SpaceX's Starshield satellite links that guide Ukrainian drones and failed to deliver air defences paid for by European allies.
The US denies deliberately hindering Ukraine's war effort.
But US President Donald Trump, who has waived sanctions on Russia in what he says is an effort to lower US fuel prices, has repeatedly rebuked Zelenskiy and called for him to be replaced.
US Secretary of State Marco Rubio acknowledged on Tuesday that a consignment of US air defence missiles bought by European countries for Ukraine may not have been delivered but denied that the US had changed policy.
"The important takeaway for you is that there has not been some deliberate political directive to go out and harm Ukraine," he told journalists during a visit to India.
The Kremlin said it was hoping to set up a meeting soon between Witkoff and Russian President Vladimir Putin, an indication that Witkoff could make the latest of several visits to the Kremlin after meeting Zelenskiy.
As US ties with Ukraine have deteriorated, Russia has stepped up attacks on Ukrainian cities, particularly targeting power stations and fuel, despite a call from Trump for both sides to spare each other's energy systems.
Russia bombarded cities in western Ukraine overnight, targeting electricity systems and knocking out power across swathes of the country that had previously been spared.
Ukraine held back for several days last week from hitting Russian oil refineries in deference to Trump's request but has resumed strikes after Russia showed no sign of complying.
Both sides have also struck data centres.
