Mr Mayers is accused of assault, alongside two others and the street-fight has become a point of contention in US-Swedish diplomacy.
On Tuesday the 30-year-old pleaded not guilty to the charge as his trial opened. He claimed to have acted in self-defense.
Robert O’Brien, the special envoy, has been sent to Stockholm by Mr Trump to monitor court proceedings and to show support for the rapper, according to Swedish news agency TT.
It is unclear why Mr O’Brien, whose job is to advise the government on hostage issues, was selected for the trip.
Dan Baer, a former US ambassador to the Organisation for Security & Cooperation in Europe, called the move a “shameful abuse of a serious office.”
“Imagine how seriously O’Brien will be taken by other actors now that he has made a mockery of himself by allowing his office to be instrumentalized thus,” he said on Twitter.
“Trump’s clown like behaviour makes us weaker.”
Suzanne Maloney, the deputy director of Foreign Policy at the Brookings Institution, described the situation as “ridiculous”.
“It’s also incredibly infuriating [especially] when there are at least a half dozen Americans sitting in Iranian prisons, some of them for years,” she said.
“What is the Trump administration doing to free Siamak & Baqr Namazi, Xiyue Wang, Michael White, Bob Levinson?”
The president had demanded that Sweden release the rapper in a series of tweets published earlier this month.
“Treat Americans fairly!”
The president also called on the Swedish government to “give A$AP Rocky his freedom”.
Mr Trump spoke to Stefan Lofven, the Swedish prime minister and offered to personally guarantee the rapper’s bail but was told Mr Lofven could not intervene in a legal case.
A senior Trump administration official, speaking on the condition of anonymity, said the special envoy’s presence at the trial was part of an effort to convey the president’s concern.
Mr O’Brien is also in Stockholm support the US citizens involved “and, to, hopefully, bring them home”, the official added.
During the trial prosecutors said 19-year-old Mustafa Jafari and a friend got into an argument with Mr Mayers and one of his bodyguards near a fast-food restaurant where the rapper’s entourage had eaten.
They allege in court documents that Mr Mayers and the two other men thought to be part of his entourage beat and kicked the teenager while he was on the ground.
Witnesses in the case are expected to testify on Thursday.
The trial continues.
Additional reporting by agencies
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