At the start of the war, Ukrainian soldiers defending Snake Island famously defied an order from a Russian warship to surrender.
The Black Sea island was seized by Russia but later reclaimed by Ukraine.
In a video, the Ukrainian president called it a "place of victory" that would never be reconquered.
In the undated clip, posted on Telegram, Mr Zelensky described it as proof that Ukraine will return every inch of its territory taken by Russia since the war began on 24 February 2022.
"I want to thank from here, from this place of victory, each of our soldiers for these 500 days," Mr Zelensky said in the video, in which he was shown arriving on the island by boat and leaving flowers at a memorial.
Last year, Russia's flagship Moskva sailed to Snake Island within hours of the start of the war and ordered Ukrainian soldiers on the island to give themselves up.
"I suggest you lay down your weapons and surrender to avoid bloodshed and needless casualties. Otherwise, you will be bombed," said a Russian officer.
The Ukrainian response and verbal exchange went viral as a soldier replied telling the Russian warship to "go to hell" - although in far cruder language.
Snake Island was seized and the Ukrainian soldiers were taken prisoner - but later exchanged for Russian captives.
Ukrainian forces re-captured the island in June last year.
Saturday's video comes after Mr Zelensky spent much of the week visiting European leaders as part of a diplomatic tour to ramp up support for Ukraine's Nato's membership.