The Institute for the Study of War says Ukraine now holds the initiative and is "in the process of securing a major victory in the region and in Kherson city".
Kherson returns to Ukrainian control and Kyiv says its military intelligence body has appealed to Russian troops who have been left behind in the area after the withdrawal, to give themselves up.
The Russian defence ministry has claimed that all Russian service personnel were moved across, saying "there were no losses of men, hardware or material of the Russian forces group.“
The Kremlin has denied its withdrawal is a failure - instead calling it a "redeployment".
How significant is the Russian withdrawal from Kherson? Watch Euronews' report in the player above.
But the Institute for the Study of War said Ukraine now holds the initiative and is "in the process of securing a major victory in the region and in Kherson city."
Kherson is the only regional capital Russia captured since its full-scale invasion of Ukraine back in February.
Russia had up to 40,000 troops in the region, with half previously stationed west of the Dnipro River, in and around Kherson city.
The only nearby road crossing from the city to the left bank of the Dnipro – the Antonivskiy bridge – has been destroyed.
The ISW also said the major Ukrainian victory underway in Kherson Oblast will not be Ukraine’s last.
"Fighting will continue on the southern axis, in Bakhmut, Donetsk region - the only place Russian forces are still attempting meaningful offensives, and in the northern Luhansk region as Ukrainian forces continue their counteroffensive operations."
The Russian campaigns to capture all these four regions - Moscow claims to have annexed all four - culminated months ago, and Russian forces are firmly on the defensive across most of the frontline.
The think tank added that Russian officials are "busy attempting to train 120,000 conscripts to deploy to the frontlines in the spring".
Ukrainian forces are likely to aim to liberate as much occupied territory as possible before those Russian reinforcements arrive, says the Institute for the Study of War.
The ISW does not believe the fighting in Ukraine will halt or enter a stalemate during the winter.
But it could disproportionately harm poorly-equipped Russian forces in Ukraine.
And well-supplied Ukrainian forces are unlikely to halt their counteroffensives due to the arrival of winter weather, and may be able to take advantage of frozen terrain to move more easily.