Elon Musk reinstated President Trump’s account on Twitter on Saturday, reversing a ban that has kept the former president off the social media site since a pro-Trump mob attacked the U.S. Capitol on January 6, 2021, as Congress was poised to certify Joe Biden’s election victory.

Mr. Musk made the announcement in the evening after holding a poll that asked Twitter users to vote on whether Mr. Trump’s account should be restored. The “yes” vote won, with 51.8 percent. Previously, Mr. Musk had said Twitter would establish new procedures and a “content moderation council” before making decisions to restore suspended accounts.

“The people have spoken. Trump will be reinstated. Vox Populi, Vox Dei,” Mr. Musk tweeted, using a Latin phrase meaning “the voice of the people, the voice of God.”

Shortly afterward, Mr. Trump’s account, which had earlier appeared as suspended, reappeared on the platform complete with his former tweets, more than 59,000 of them. His followers were gone, at least initially, but he quickly began regaining them. There were no new tweets from the account as of Sunday morning, however.

Many prominent left-wing groups were incensed by Mr. Musk’s decision. Jonathan Greenblatt of the Anti-Defamation League called the decision “dangerous and a threat to American democracy” on — of course — Twitter.

“For @elonmusk to allow Donald Trump back on Twitter, ostensibly after a brief poll, shows he is not remotely serious about safeguarding the platform from hate, harassment and misinformation,” Mr. Greenblatt said.

In a statement, the NAACP called on advertisers to abandon the platform. “If Elon Musk continues to run Twitter like this, using garbage polls that do not represent the American people and the needs of our democracy, God help us all,” the organization said.

On Friday, Mr. Musk tweeted that the suspended Twitter accounts for the comedian Kathy Griffin, the Canadian psychologist Jordan Peterson and the conservative Christian news satire website Babylon Bee had been reinstated. He also responded “no” when someone on Twitter asked him to reinstate the conspiracy theorist Alex Jones’ account.

Mr. Peterson was banned from Twitter in June for violating the site’s “hateful conduct policy” by using the wrong gender of an actress who changed her gender identity, and the Babylon Bee was banned in March for the same reason after it referred to a transgender Biden administration official, Rachel Levine, as “Man of the Year.” Ms. Griffin was banned on November 6 for impersonating Mr. Musk on the site and beseeching followers to vote Democratic.

Mr. Musk restored Mr. Trump’s account less than a month after the Tesla CEO took control of Twitter and four days after Mr. Trump announced his candidacy for the 2024 presidential race.

It is not clear whether Mr. Trump would actually return to Twitter. An irrepressible tweeter before he was banned, Mr. Trump has said in the past that he would not rejoin even if his account was reinstated. He has been relying on his own, much smaller social media site, Truth Social, which he launched after being blocked from Twitter.

And on Saturday, during a video speech to a Republican Jewish group meeting in Las Vegas, Mr. Trump said that he was aware of Mr. Musk’s poll but that he saw “a lot of problems at Twitter.”

“I hear we’re getting a big vote to also go back on Twitter. I don’t see it because I don’t see any reason for it,” Mr. Trump said. “It may make it, it may not make it,” he added, apparently referring to Twitter’s recent internal upheavals.

The prospect of restoring Trump’s presence to the platform follows Musk’s purchase last month of Twitter — an acquisition that has fanned widespread concern that the billionaire owner will allow purveyors of lies and misinformation to flourish on the site. Mr. Musk has frequently expressed his belief that Twitter had become too restrictive of freewheeling speech.

Mr. Musk’s online survey, posted on his own Twitter account, drew more than 15 million votes in the 24 hours in which it ran.

Mr. Trump lost his access to Twitter two days after his supporters stormed the Capitol, soon after the former president had exhorted them to “fight like hell.” Twitter dropped his account after Trump wrote a pair of tweets that the company said cast further doubts on the legitimacy of the presidential election and raised risks for the Biden presidential inauguration.

After the January 6 attack, Mr. Trump was also kicked off Facebook and Instagram, which are owned by Meta Platforms, and Snapchat. His ability to post videos to his YouTube channel was also suspended. Facebook is set to reconsider Mr. Trump’s suspension in January.