After less than a month in exile, the white nationalist leader Richard B. Spencer is back on Twitter.

The social network reinstated Mr. Spencer’s personal account on Saturday, less than four weeks after suspending it and others he created for violating rules against making multiple accounts with overlapping uses.

“Please select one account for restoration; the others will remain suspended,” the company told Mr. Spencer last week, according to an email that was obtained by BuzzFeed. The account was suspended on Nov. 15, and a spokesman for Twitter said in an email that it first contacted Mr. Spencer three days later, on Nov. 18, and again last week.

Mr. Spencer, a leader of the so-called alt-right, a far-right fringe movement that embraces white nationalism, racist, anti-immigrant and anti-feminist positions, celebrated his return on Saturday night by writing “I’m back,” accompanied by a clip of Arnold Schwarzenegger from the “Terminator” movie franchise.