When former Representative George Santos, the serial fabulist, was expelled from Congress by his colleagues in December, he left in a huff, declaring, “to hell with this place.”

On Thursday night, the Tom Ripley of Congress was back.

Dressed in a crystal-encrusted collar, Mr. Santos took a seat on the House floor in a surprise appearance ahead of President Biden’s State of the Union address, putting to use the lifetime floor privileges conferred on former members of the House — even the ones who are expelled.

There, he was greeted more warmly than he was ever treated when his colleagues wanted nothing to do with him.

On Thursday night, he sat and chuckled with Representatives Lauren Boebert of Colorado and Matt Gaetz of Florida. Representative Marjorie Taylor Greene of Georgia greeted him with a warm embrace. Even members who had been publicly critical of him in the past, like Representative Claudia Tenney of New York, stopped to snap his picture, while others made a beeline to greet him.

It was fair to say that the disgraced former congressman, who is scheduled to go on trial in September on federal fraud charges that include accusations of stealing money from campaign donors for personal expenses, was doing something akin to holding court.

Since leaving Congress, Mr. Santos has tried to launch a career on the video app Cameo, monetizing on his strange slice of fame by charging hundreds of dollars a pop for his short personalized videos. But interest in micro-celebrities can be fleeting.

And he had been carefully planning his splashy return. Mr. Santos told some members of the press corps that he planned to return on Thursday night to seize back the spotlight, but swore them to secrecy.

It was not clear what high jinks he had planned, aside from the fact of his presence alone. At last year’s State of the Union address, Mr. Santos got in a confrontation with Senator Mitt Romney, Republican of Utah, who told him, bluntly, “you don’t belong here.” Mr. Santos had stationed himself close enough to the action to reach out for a presidential handshake.

Speaker Mike Johnson earlier Thursday tried to encourage members to show decorum during Mr. Biden’s speech.

Not everyone was willing to make that promise.

Mr. Santos appeared to be staking out a seat near the corridor where Mr. Biden was set to enter the chamber, along with some of the hard-right members of the Republican conference, such as Ms. Boebert.

“Oh, you think I plan that stuff?” Ms. Boebert said, when asked how she planned to behave during the speech. “I’m more spontaneous than you think.”

But minutes ahead of the speech, without a seat saved, Mr. Santos moved himself to the back of the chamber.