The killing occurred around 7:30 p.m. when the suspect gave the victim, who FOX affiliate WJBK-TV reported was a panhandler, some money before walking away.
The shooter then walked back to the victim and shot him once and fled, police said.
The victim was pronounced dead at the scene. Authorities have not disclosed a motive for the killing.
"A lot of our homicides are people that know each other, maybe domestic incidents, but in this case, it just seems like a random execution for no reason and it's highly concerning for us," Detroit Police Capt. James McGinnis told the news outlet.
The station said the victim was a man in his 50s who frequently hung out near the gas station.
The suspected shooter is described as White, standing five feet, nine inches tall.