The former US president delivered a two-and-a-half minute speech at a wedding reception held at his Mar-a-Lago resort in Florida over the weekend.
In a video released by TMZ, Mr Trump is heard saying: "I get all these splash reports, and they're telling me about the border, they're telling me about China, they're telling me about Iran."
The 74-year-old suggested the President Biden had ended sanctions in Iran: "We were ready to make a deal, they were ready to do anything, they would have done anything. And this guy goes and drops the sanctions and then he says we'd like to negotiate now."
The Biden administration has not dropped any sanctions against Iran.
Mr Trump then moved onto the issue of immigration.
"The border is not good. It's the worst that it has ever been," he said.
"What's happening to the kids, they're living in squalor. They are living like nobody has ever seen. There's never been anything like this."
With the wedding band in the background of his speech - looking as if they were waiting to play - Mr Trump then attacked the US election result.
"They said get 66 million votes, sir, and the election is over. I got 75 million votes, and you saw what happened," he said.
Joe Biden was the first US presidential candidate to win more than 80 million votes - winning the popular vote and the electoral college.
Mr Trump concluded his speech by toasting the bride and groom.
"I just want to say, it's an honour to be here. It's an honour to have you at Mar-a-Lago. You are a great and beautiful couple."