It happened about 3:50 p.m. in the 300 block of S. Elm Street.
Witnesses told police there was an argument, followed by gunfire.
The victim, identified as 34-year-old Michael Lorenzo Joyner, of Columbia, South Carolina, was found face down in the road with a gunshot wound to the head.
Police Chief Tyrone Sutton said the victim's body armor indicates he had clearly tried to prepare for something before he arrived in Princeton.
Family members told police that Joyner traveled to Princeton from South Carolina to check on his brother. Sutton said the victim's brother had been in an argument earlier Monday with three suspects in the shooting
Police said Tuesday that it is not confirmed as to the purpose or intent of Joyner's visit to Princeton.
No arrests have been made. There are persons of interest identified, police told ABC11.
The incident remains under investigation.