Mr Biden was singled out over his position in Westminster Abbey's south transept.
Mr Trump posted a photograph of the assembled world leaders, with a red arrow pointing to Mr Biden.
The former US President said that, had he still been in office, he would have been seated closer to the front.
Writing on Truth Social – his social media platform – he argued that it showed the standing of the US had fallen during Mr Biden's tenure as president.
US President Joe Biden in Westminster Abbey
POOL
Joe and Jill Biden being shown to their seats
POOL
He said: "This is what’s happened to America in just two short years. No respect!
"However, a good time for our President to get to know the leaders of certain Third World countries.
"If I were president, they wouldn’t have sat me back there – and our Country would be much different than it is right now!
"In Real Estate, like in Politics and in Life, LOCATION IS EVERYTHING!!!"
Mr Biden's position in the ancient abbey was primarily due to protocol, as Commonwealth leaders are placed in front of those from other countries.
That meant Justin Trudeau, the Prime Minister of Canada, was nine rows in front of the US President.
Mr Biden and First Lady Jill Biden were seated next to Ignazio Cassis, the president of Switzerland.
Andrzej Duda, the Polish president, was directly in front of the Bidens and immediately behind them was Petr Fiala, the Czech prime minister.
China’s representative, Wang Qishan, was seated in the same row.
It remains unclear which leaders Mr Trump meant when he referred to "Third World countries".