A photo of the first daughter sitting next to Chinese President Xi Jinping, British Prime Minister Theresa May, Turkey's Recep Tayyip Erdogan and German Chancellor Angela Merkel was first tweeted by Svetlana Lukash, who identifies herself on Twitter as a G20 Russian sherpa, a title given to people who help international delegations at large summits.
A senior Trump administration official confirmed that Ivanka Trump had stepped in for her father, but dismissed any suggestion it was improper or unusual.
"Ivanka was sitting in the back and then briefly joined the main table when the President had to step out, and the president of the World Bank started talking as the topic involved areas such as African development -- areas that will benefit from the facility just announced by the World Bank," the official said, referencing a initiative Ivanka Trump has spearheaded that looks to fund women entrepreneurs.
"When other leaders stepped out, their seats were also briefly filled by others," the official added.
Brian Fallon, a CNN political commentator and former spokesman for Hillary Clinton's presidential campaign, responded to the news with a tweet: "I'm sure Republicans would have taken it in stride if Chelsea Clinton was deputized to perform head of state duties," he wrote.
Photos from the room show Ivanka Trump -- along with Treasury Secretary Steven Mnuchin -- seated rows behind the President during the portion of the meeting he attended. Aides and advisers regularly attend G20 meetings with world leaders, as Ivanka Trump, Mnuchin and others did during Friday's session.
Merkel, asked about Ivanka Trump's sitting in for the President, said, "The delegations themselves decide" who takes the seat if the principal isn't there.
"Ivanka Trump was part of the American delegation, so that is something that other delegations also do, and it is very well known that she works in the White House and that she is also engaged in certain initiatives," Merkel told reporters at a news conference.
But Michael McFaul, the United States ambassador to Russia under President Barack Obama, said Ivanka Trump subbing in was unusual.
"This is strange," he tweeted. "Very strange."
Brian Klaas, a Trump critic and fellow at the London School of Economics, tweeted the photo of Ivanka Trump, adding, "Ivanka Trump, unelected, unqualified, daughter-in-chief, is representing the US at the G20 summit next to May, Xi, Merkel."
Donald Trump -- both as a businessman and as President -- has regularly blended his family with official duties. Jared Kushner, Ivanka Trump's husband, is also one of Trump's top advisers, for example.
The President, shortly before the working session where his daughter briefly subbed in for him, showered praise on Ivanka Trump at an event less than an hour earlier.
"If she weren't my daughter, it would be so much easier for her," Trump said, seemingly suggesting that his controversial personality is negatively impacting his less controversial daughter. "Might be the only bad thing she has going, if you want to know the truth."
Ivanka, along with World Bank President Jim Yong Kim and Merkel, helped launch the World Bank-backed program that will promote women's entrepreneurship in developing countries.
"I am very proud of my daughter Ivanka," Trump said. "Always have been, from day one, I have to tell you that, from day one, she has always been great."