Donald Trump reached the RNC and the Fiserv Forum, typically the home of the Milwaukee Bucks basketball team, at roughly 9 p.m. local time (0200 GMT/UTC).
He had a medical gauze covering most of his right ear.
He arrived as a speaker was leaving the stage and as the band was filling a gap in proceedings.
They played the Lee Greenwood song "God Bless the USA" after a brief introduction for Trump, who did not speak but gestured and pumped his fist as the big screens of the venue focused on him.
"Ladies and gentleman, the 45th President of the United States, and soon to be the 47th President of the United States, please welcome Donald J. Trump," the singer told the crowd before embarking on the song's last chorus.
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Lee Greenwood gives an incredible introduction to Donald Trump as he enters the RNC Convention
Biden says he will debate Trump again in September
US President Joe Biden has said in an interview aired on US broadcaster NBC that he will take part in the second scheduled presidential debate against Donald Trump in September.
"I'm gonna debate him when we agreed to debate. And I agreed to debate him in September," Biden said.
Biden has faced pressure since his performance in the first debate last month and increased scrutiny at public appearances since. He was asked about this during the interview.
"Look, 14 million people voted for me to be the nominee, of the Democratic Party," Biden said, referring to the results of Democratic primaries and caucus. "I listen to them."
Asked if he'd since seen the debate in full, he said "I've seen pieces of it but not the full debate."