Confusingly, the post was deleted from Instagram after around half an hour – only to be reposted a short time later.
The video showed a group led by controversial doctor and preacher Stella Immanuel standing on the steps of the Supreme Court, making the unsubstantiated claim that hydroxychloroquine is a "cure for COVID" and that neither masks nor lockdowns are necessary to control the spread of the virus.
The "Deliverance Minister" said she went to medical school in Nigeria where she treated malaria patients with the drug, which US President Donald Trump claimed could prevent or treat COVID-19. Mr Trump had previously shared the same clip on Twitter, declaring it a "must-watch".
Madonna, who claims to have contracted coronavirus herself in March, wrote: "Some people don't want to hear the truth. Especially the people in power who stand to make money from this long drawn-out search for a vaccine which has been found and proven and has been available for months. They would rather let fear control the people and let the rich get richer and the poor and sick get sicker."
Madonna finished by declaring that Stella Immanuel – who has a history of making bizarre medical claims involving alien DNA, witchcraft and the illuminati, and is preacher of a church with an anti-gay stance – was her "hero".
Immanuel's own Facebook page describes her as a "physician, author, speaker, entrepreneur, deliverance minister, God's battle-axe and weapon of war".
The initial post stayed up on Madonna's Instagram account for around half an hour, as fans flooded the comments begging her to delete it or research her "hero" in more depth.
But after Madonna – or, perhaps, Instagram – deleted the initial post, it soon reappeared on her page again.
"Are you kidding? Has your Instagram been hijacked?" one fan wrote.
"You need to take this down. This full of misinformation," said another.
"MADONNA!!! Again?! This is wrong! You have influence and a voice and you're spreading falsehood and quackery. Inform yourself! Or, as you say, WAKE UP!!!" wrote another as she reposted the video.
It's the latest in a series of Instagram faux-pas for the singer during lockdown, starting with a bizarre video she posted in March of herself nude and pouting in a rose petal-filled bath, telling her followers what was "wonderful" about the pandemic.
