The Chinese leader has told Trump previously that Beijing expects Washington to continue managing relations based on the "One China" principle that rules out formal contacts with Taiwan, which China claims as its own territory.
Foreign ministry spokesman Geng Shuang said China asked the United States for an explanation of the mistake, and the US said it was a technical error.
Call it the Trump affect or something else, but major anomalies have been emerging from the White House lately.
Chinese President Xi met his U.S. counterpart Donald Trump after the closing ceremony of the G20 Hamburg Summit, holding talks on economic plans and the upcoming meeting between the countries' military officials.
An official online version of the release now simply refers to Xi as: "President Xi of China".
"What a bunch of amateurs", China expert Bill Bishop tweeted in response to the faux pas.
All political watchers will know that China is especially allergic to any questioning of their "One China" policy in relations to China-Taiwan ties.
Xi's correct title should be the President of the People's Republic of China.
Xi, striking a more sober note than Trump, said "there is a lot of work needed to be done" on the "sensitive issues [that] remain in the China-U.S. relationship".
The meeting between Trump and Xi took place in Hamburg and spanned 90 minutes, with the threats of North Korea's nuclear program and bilateral trades between the two countries being on the agenda, reports said.