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.”