Ukraine's parliament published pictures of people carrying Ukrainian flags in the center of Kherson, the capital of the eponymous region and close to the Black Sea.
"Kherson is returning to Ukrainian control and units of the Armed Forces of Ukraine are entering the city," Kyiv's defence ministry said on social media.
It added that its artillery teams had clear views over Russia's routes to retreat and warned: "Any attempts to oppose the Armed Forces of Ukraine will be stopped."
Kyiv's announcement followed one from Moscow saying its forces had left the key city.
"Today at 5 o'clock in the morning Moscow time (0200 GMT), the transfer of Russian troops to the left bank of the Dnipro River was completed. Not a single piece of military equipment and weapons was left on the right bank," the Russian defense ministry said in a statement on social media.
The announcement came hours after Russian strikes battered a residential building in Mykolaiv, killing at least seven people in the city near Kherson that Moscow's forces have been battering for months.
The Kremlin meanwhile insisted that the city was still part of Russia and said it did not regret annexing the urban hub and the Kherson region at a lavish ceremony in late September.
Ukrainian officials have remained wary after Moscow announced this week that it would pull forces to defensive positions on the east bank of the river in Kherson, in what would be a major Russian setback in a region Vladimir Putin claimed to have annexed.
The Kremlin however on Friday dismissed any suggestion the status of the region had changed after the retreat.
"This is a subject of the Russian Federation. There are no changes in this and there cannot be changes," Kremlin spokesman Dmitry Peskov told reporters.
The region of Kherson was one of four territories of Ukraine that Vladimir Putin annexed during a grand ceremony in the Kremlin in late September, at the same time vowing to use all available methods to defend it from Kyiv.
Kherson was the first major urban hub to fall to Russian troops after President Vladimir Putin announced Moscow's "special military operation" in Ukraine, and it was the only regional capital his forces seized.
Its capture by Ukraine's forces would be a political and symbolic blow to Putin, but it would open a gateway for Ukraine's forces to the entire Kherson region with access to both the Black Sea in the east and Sea of Azov in the West.
It would also disrupt an important land bridge for Russia between its mainland and the Crimean peninsula, which Moscow annexed from Ukraine in 2014.