The trip marks his first official appearance outside the Kyiv region since the start of Russia's invasion on February 24.
The president's office website said Zelenskiy spoke to soldiers, handing out commendations and gifts.
Several explosions were heard in the city of Kharkiv, just hours after the president's visit, according to a Reuters journalist.
Kharkiv has been subjected to Russian shelling in recent days after several weeks of relative quiet.
Russian troops have instead been advancing in the eastern parts of Ukraine.
Invading forces appear close to seizing all of the Luhansk region of Donbas, one of the more modest war goals the Kremlin set after abandoning its assault on Kyiv in the face of Ukrainian resistance.
Armored Ukrainian vehicles were present near Bakhmut in the eastern Donetsk area on Sunday (May 29).
Authorities had reported Russian artillery was shelling the Lysychansk-Bakhmut road, which Russia must take to encircle Ukrainian forces in the region.
Zelenskiy spoke of the tension in the east on Saturday.
"The situation is very complicated. Especially in those areas in the Donbas and Kharkiv region where the Russian army is trying to squeeze at least some result for themselves.
"But our defenses are holding up. It's indescribably difficult there. And I am grateful to all those who withstood this onslaught of the occupiers."
Russia says it is waging a "special military operation" to demilitarize Ukraine and rid it of nationalists threatening Russian-speakers there.
Ukraine and Western countries say these claims are a false pretext for a war of aggression.
