The vessel, which is on its way from Hong Kong, belongs to Alexei Mordashov, an ally of Russia's Vladimir Putin who has been sanctioned by the EU and US.
Comment: According to the mainstream media, everyone Russian who isn't a Western asset is an 'ally of Putin'.
The South African government says it is not bound by the sanctions.
But Cape Town's mayor says he has a "moral duty" to oppose Russia's "unjust war" and has vowed to block the yacht.
Comment: The mayor's moral duty is to do some reading about about the real reasons for Russia's just response to the West's provocations.
Mayor Geordin Hill-Lewis is a member of the opposition Democratic Alliance, which has urged the government to seize the 465ft (141m) yacht.
It is not clear how Cape Town officials plan to stop the vessel - called The Nord - from docking.
A spokesman for President Cyril Ramaphosa said: "South Africa has no legal obligation to abide by sanctions imposed by the US and EU."
"South Africa's obligations with respect to sanctions relate only to those that are specifically adopted by the United Nations," the spokesman, Vincent Magwenya, noted.
Accusing Mr Geordin Hill-Lewis of "grandstanding", he added: "Municipal governments have no legal control over the country's borders."
Hong Kong's government recently offered similar justifications when refusing to prevent the entry of the $521m (£472m) superyacht, with Chief Executive John Lee saying the city would be accountable to UN sanctions.
Western countries and their allies have imposed sanctions on more than 1,000 Russian individuals and businesses in response to Russia's invasion of Ukraine.
Mr Ramaphosa's government has so far avoided directly criticising Russia, abstaining in several UN votes that have expressly condemned the war.
Comment: Ramaphosa probably hasn't 'indirectly' criticised Russia, either.
Pretoria has also called for a negotiated settlement to end the conflict.
As in many other African countries, South Africa's leadership has ties to Russia dating back to the fight against colonial rule.
Comment: Uganda's officials recently praised Russia for their help in decolonizing their country from the West.
On Monday, Mr Geordin Hill-Lewis wrote on Twitter that there could be "no place in our city for accomplices to, and enablers of, Putin's war".
"Our country's foreign policy conduct in relation to Russia's illegal, imperialist war has been nothing less than shameful," he added in a further statement. "Here is an opportunity to correct some of those errors of judgement and stand up for what is clearly right."
Comment: As the President said, the mayor is grandstanding.
The luxury yacht is expected to arrive in Cape Town in the next few weeks, it is not known how long it will be allowed to stay.
Prior to the war, Mr Mordashov was Russia's wealthiest man. The 57-year-old built a fortune of some $29.1bn (£25.92bn) through his steel and mining company Severstal.
Early in the conflict he was targeted by extensive Western sanctions because of his links with Mr Putin. But the billionaire says he is not involved in Russian politics.
Mr Mordashov has already lost one of his smaller vessels, the Lady M, to Western sanctions after it was seized by Italian police in March.
But The Nord is believed to be his biggest yacht. Forbes magazine describes it as one of the world's most extravagant boats.
Shortly after the outbreak of the conflict in Ukraine, the yacht - which features two helicopter pads, a swimming pool and a cinema - left the Seychelles for the far-eastern Russian port of Vladivostok. The move was seen as a bid to avoid the fate of the Lady M.
But experts say owners like Mr Mordashov face serious issues finding international ports capable of maintaining superyachts. Most are in Europe, where the vessels would be instantly seized.




the "south african gubment", whatever the hell that is had assets on the yacht and the russian oligarch laid it all out for them....so, they say the boat gets a free pass, and at the end of the day it was a win-win.
Could of got real ugly is what they said after the verdict came down.
Poem of the Day, from a nice place where the best yacht in town is a rowboat gently swaying.....102622 522 pm...swaying in the river...old river it is - must be some gold there!