The meeting was cordial. But the first lady's clothing choices — specifically her decision to cover her head at the Vatican in contrast to during visits to Saudi Arabia and Israel — drew commentary from many sources.
Some criticized her for not covering her hair during a visit to the Western Wall, part of a destroyed Jewish temple complex, or while in Saudi Arabia.
Others praised Trump for not covering up in Saudi Arabia, seeing the move as a nod to women's empowerment, which she promoted on the trip.
Despite the criticism, the first lady's attire was in keeping with the protocol of the countries she visited and the precedents set by foreign leaders who have visited them.
Vatican protocol dictates that women should wear long sleeves, formal black clothing and a black veil during private audiences with the pope.
The United Kingdom's Queen Elizabeth II attracted attention in 2014 for wearing a lilac suit to meet Pope Francis. That acceptance was seen as an effort by the pope to relax some rules of the Vatican. There is also an exception to black granted to Catholic queens or Catholic spouses of kings, according to Vatican Insider.
Camilla, Duchess of Cornwall, wore cream and did not cover her head when meeting Francis last month, although she had worn black and a veil at a 2009 meeting with Pope Benedict XVI.
"Things have become more relaxed over the last few years. There are no hard and fast rules," a Vatican spokesperson told British newspaper the Daily Express.
At the Western Wall, the first family did follow Jewish religious custom. The men (President Trump and son-in-law Jared Kushner) went to the men's prayer plaza while the president's daughter Ivanka and the first lady went to the women's area, Israeli newspaper Haaretz reported.
Ivanka Trump, who converted to Judaism, wore a hat at the Western Wall as it is customary for observant women to cover their hair.
Saudi Arabia, where Melania Trump went veil-less, has a strict dress code for women — Muslim women are to wear a full-length abaya robe and headscarf. However, foreigners are considered exempt from that rule, according to the Economist.
As Fox News noted, in 2015, Donald Trump criticized former first lady Michelle Obama on Twitter for not wearing a headscarf on a trip to Saudi Arabia.
British Prime Minister Theresa May visited Saudi Arabia last month sans headscarf as well.
Melania Trump's spokeswoman told CNN that the black dress with long, flowing sleeves she wore in Saudi Arabia was chosen to resemble the country's abayas.
"If you look at her style, it's still very much her and her style of dress but being sensitive to the place and host nation for sure," Stephanie Grisham said.