"My pronouns are Prosecute/Fauci," the billionaire Twitter CEO said, alluding to the practice of indicating gender pronouns after one's name as well as the right-wing campaign to charge Fauci with crimes related to his involvement in US Covid policies.
Musk also posted a meme showing Fauci telling US President Joe Biden, "Just one more lockdown, my king..." - in apparent criticism of the Covid mitigation measure Musk has repeatedly slammed but has not been deployed in the country for over a year.
Early in the pandemic, Musk tweeted that concern over the virus was "dumb" and since taking over Twitter has removed its policy targeting Covid misinformation.
Musk's tweet quickly went viral, receiving over 800,000 likes within some 11 hours but also sparking swift criticism.
Vaccine scientist and author Peter Hotez called on Musk to delete the tweet, saying, "200,000 Americans needlessly lost their lives from Covid due to this kind of antiscience rhetoric and disinformation."
Democratic Senator Amy Klobuchar praised how Fauci "calmly guided our country through crisis" and addressed Musk, saying: "Could you just leave a good man alone in your seemingly endless quest for attention?"
But Musk received praise from right-wing corners.
Republican lawmakers have pledged to grill Fauci when they take control of the House of Representatives in January, after locking horns repeatedly with the top immunologist over Covid vaccines, mask mandates and other pandemic-related issues.
Fauci, 81, is due to step down this month from his roles in government as Biden's chief medical advisor, as well as director of the National Institute for Allergies and Infectious Diseases, which he has headed since 1984.
In what was likely his final White House appearance in November, Fauci slammed the proliferation of bad health advice online and said the most difficult thing he had to deal with while helming America's fight against Covid was the country's polarization along political lines.