Mr Trump said he was “very disappointed in Prime Minister Stefan Lofven,” whom he personally lobbied, “for being unable to act”.
Mr Trump also called on Sweden to “Treat Americans fairly!” and “Give A$AP Rocky his FREEDOM,” adding: “We do so much for Sweden but it doesn’t seem to work the other way around.”
He used the popular hashtag “#FreeRocky.”
However, fresh details about the case have emerged, after Swedish authorities released court documents alleging Mr Mayers and two other men “deliberately, together and in agreement” assaulted the victim in central Stockholm.
Prosecutors alleged the victim was punched and kicked while on the ground and attacked with a bottle.
The 500-page document also included photos of the victim’s injuries, mostly cuts and bruises, and blood-stained clothes.
The charges can carry a maximum sentence of two years in prison.
A$AP Rocky is a platinum-selling, Grammy-nominated artist who has collaborated with Kendrick Lamar, Cardi B, Drake and Selena Gomez.
He has been in custody since July 3 as authorities investigate a fight he was allegedly involved in on June 30 before appearing at a music festival.
He and the two other suspects - who have been described as members of his entourage - will remain in custody until trial.
Prosecutors have recommended that the Stockholm District Court set aside three days for the trial, which defence lawyer Slobodan Jovicic said is expected to start on Tuesday.