Zelensky published on social media an undated video clip of a visit to Snake Island in the Black Sea -- a symbol of Ukraine's defiance against Russia.
"Today we are on Snake Island, which will never be conquered by the occupiers, like the whole of Ukraine, because we are the country of the brave," he said.
"I want to thank from here, from this place of victory, each of our soldiers for these 500 days," he said in the video, which showed him arriving on the island by boat and leaving flowers.
The UN has documented 9,000 civilian deaths since the start of the war on February 24, 2022, including 500 children, although it estimates the real toll could be significantly higher.
That toll rose again on Saturday as Ukraine's interior ministry said eight people were killed and 13 injured by Russian rocket fire in the eastern town of Lyman.
Russian news agencies reported that two civilians were killed by Ukrainian shelling in the Russian-held town of Oleshky in the south.
Noel Calhoun, deputy head of the UN's Human Rights Monitoring Mission in Ukraine, said the 500th day of the conflict marked "another grim milestone in the war that continues to exact a horrific toll on Ukraine's civilians".
- 'Act of desperation' -
Zelensky on Saturday completed a visit to Turkey in which he secured backing for his country's NATO aspirations after winning a US pledge for cluster munitions that could inflict significant damage to Russian forces on the battlefield.
On his return to Ukraine, Zelensky also brought back five Ukrainian commanders who were supposed to have remained in Turkey until the end of the conflict under a prisoner exchange deal.
Russia denounced the release of the men, members of the crack Azov regiment celebrated in Ukraine for their stiff resistance at the Azovstal factory during the months-long siege of the port city of Mariupol.
"The return of Azov commanders from Turkey to Ukraine is nothing but a direct violation of the terms of existing agreements," Kremlin spokesman Dmitry Peskov was quoted as saying by Russian news agencies.
He said Ukraine and Turkey had violated the terms of the agreement and claimed their return was linked to the "failure of the counter-offensive" launched by Ukraine.
Washington's decision to supply Ukraine with cluster bombs -- banned across a large part of the world -- dramatically ups the stakes in the war.
US President Joe Biden admitted that supplying Ukraine with weapons that are capable of covering several football fields with hundreds of multiple small explosives was "a difficult decision".
Humanitarian groups strongly condemned the decision to supply cluster munitions, which can go undetonated and potentially endanger civilians for years to come.
Russia, which itself uses cluster munitions in Ukraine, also criticised the decision, saying it was an "act of desperation" that would have "no effect" on the conflict.
Russian foreign ministry spokeswoman Maria Zakharova also said Ukraine's assurances it would use the ammunition responsibly "are not worth anything".
- 'Contact is possible' -
Various international efforts to mediate in the conflict have so far failed.
Turkish President Recep Tayyip Erdogan has tried to portray himself as a neutral mediator, substantially boosting wartime trade with Russia while supplying Ukraine with drones and other weapons.
But while reaffirming his longstanding call for both sides to enter peace talks, Erdogan risked drawing Russia's ire by delivering unequivocal support for Ukraine's NATO aspiration.
"There is no doubt that Ukraine deserves membership of NATO," Erdogan told reporters in Istanbul.
Erdogan also said he would meet Russian President Vladimir Putin next month on what would be the Kremlin chief's first visit to Turkey since the invasion.
Asked about a possible meeting with Erdogan, Kremlin spokesman Peskov was quoted on Saturday as saying: "Contact is possible. There are no fixed dates yet."
- Symbol of defiance -
Zelensky meanwhile vowed defiance in the video of him visiting Snake Island, which was captured by Russian forces shortly after the start of the invasion.
A radio exchange went viral in which Ukrainian soldiers told the crew of Russia's attacking warship demanding their surrender to "go fuck yourself".
The Ukrainian soldiers were taken prisoner but later exchanged for Russian captives.
The Russian ship involved, the Moskva, sank in the Black Sea in April 2022 following what Moscow said was an explosion on board.
Ukraine said it had hit the warship with missiles.
Ukrainian forces recaptured the island in June last year.