Access to the network in the country has been blocked since Saturday morning, hours after judge Alexandre de Moraes ordered the suspension of the platform.
Moraes's ruling came after Musk failed to comply with an order to name a new legal representative for the company in Brazil -- a requirement X has rejected as "illegal."
Moraes and Musk have been locked in a high-profile feud for months as the judge has vowed to fight disinformation and Musk claims the supremacy of free speech.
On Monday, five Supreme Court judges, including Moraes himself, voted in a virtual session to uphold his ruling.
"Elon Musk demonstrated his total disrespect for Brazilian sovereignty and, in particular, the judiciary, positioning himself as a true supranational entity, immune to the laws of countries," Moraes said in the new ruling.
His colleague Flavio Dino added that "freedom of expression does not excuse repeated violations of the legal system."
Musk, who also owns Tesla and SpaceX, reacted with fury to last Friday's order, branding Moraes as an "evil dictator cosplaying as a judge" and accusing him of "trying to destroy democracy in Brazil."
The standoff began when Moraes ordered the suspension of several X accounts belonging to supporters of Brazil's former far-right president Jair Bolsonaro.
Bolsonaro had tried to discredit the voting systems used during the country's 2022 elections, which he lost.
Brazilian authorities are investigating whether Bolsonaro plotted a coup attempt to prevent his successor, Luiz Inacio Lula da Silva, from assuming office in January 2023.
Online users blocked by Moraes include far-right ex-congressman Daniel Silveira, who was sentenced to nine years in prison in 2022 on charges of leading a movement to overthrow the Supreme Court.
In April, Moraes ordered an investigation of Musk, accusing him of reactivating some of the banned accounts.
Musk is also the subject of a separate investigation into the alleged use of public money to orchestrate disinformation campaigns in favor of Bolsonaro and those close to him.
The suspension of X came just over a month before municipal elections are to be held in Brazil.
In August, X announced it was shuttering its offices in Brazil due to the Moraes's actions, but assured clients they would still have access to its services.
Until Friday, X counted about 22 million users in Brazil, according to the site DataReportal.
