The 13 border guards were initially reported dead after refusing to surrender to Russian forces and swearing at them.
Ukraine's Navy has said that 13 border guards defending Snake Island are alive and have been captured by Russia.
The men had initially been reported dead last week, but Ukrainian authorities now say they were "captured by the Russian occupiers".
"We are very happy to learn that our brothers are alive and well," the Ukrainian Navy said on its official Facebook page.
"The Marines and frontier guards [were] captured by the Russian occupiers on Snake Island."
Chaotic information surrounding the Russian invasion of Ukraine has led to confusing and contradictory accounts.
On Friday, Ukrainian officials reported that all troops stationed on the strategic Snake Island (also known as Zmiinyi Island) in the Black Sea had been killed.
The men could be heard defiantly refusing Russian demands to surrender, prompting international attention.
Viral audio clips showed the border guards were told by Moscow forces to "lay down their weapons and surrender to avoid bloodshed and needless casualties" or "be bombed".
The Ukrainian soldiers could then be heard speaking amongst themselves, before swearing at their opponents and telling the Russian warship to "go fuck yourself".
After reports of their deaths, Ukraine's President Volodymyr Zelenskyy awarded each of the guards the posthumous title of "Hero of Ukraine".
“All border guards died heroically but did not give up,” he said.
Russia had denied allegations that the 13 Ukrainian soldiers were killed by air and sea strikes. Russian state TV later showed what it claimed was footage of the soldiers alive in custody, but the footage could not be independently verified.
The State Border Guard Service of Ukraine (SBGSU) had also u-turned and suggested that the 13 border guards may still be alive.
Kyiv said that the island's communications and infrastructure had been "completely destroyed" by Russian "occupiers," making it harder to find out what had happened to the soldiers.
Ukraine's Navy says that a humanitarian mission that had been sent to Snake Island after the soldiers reported deaths has also been captured.
"The illegal seizure of a non-combatant civilian ship that did not carry out any military tasks is a violation of the rules and customs of war and international humanitarian law," the Navy said.
"We demand from the Russian Federation immediate release of illegally captured citizens of Ukraine."