Calvin Riley and a friend were playing the game in the Fisherman's Wharf area, along San Francisco's waterfront, when Riley was shot, AP reported.
According to family and friends, Riley was randomly targeted, and there was no confrontation before the attack, KGO-TV reported.
"There was nothing said back and forth,” family friend John Kirby told KGO-TV. "It was just senseless, just came up and shot in the back and ran away for nothing."
According to Kirby, Riley and his friend saw someone watching them from the top of a hill, but they were playing the game, so they were absorbed in their cellphones.
Calvin was ahead of his friend, according to Riley, who said the friend witnessed the end of the attack.
“Then when the friend came around a corner he heard a gunshot, saw his friend fall, and whoever did it ran away and possibly got into a car," Kirby told KGO-TV said.
The gunman did not take any of Riley’s possessions, Sgt. Robert Jansing, a detective with the
U.S. Park Police, told AP.
Investigators believe there are other witnesses because the crime took place in a busy area, AP reported.
A GoFundMe account set up on behalf of Riley's family has already raised over $35,000 to pay for funeral expenses.
Riley's cousin, Gabriel Antonio Morales said the family learned about Riley's death around 4:30 a.m., according to a statement on the GoFundMe page.
He said Riley, who played college baseball, had dreamed of one day playing in the major leagues.
"You had all the mechanics, talent and dedication to make your dreams a reality," Riley's cousin, Gabriel Antonio Morales wrote on the GoFundMe page. "But all that was taken by a coward who wanted to shoot up a poké stop."
Pokémon Go is an augmented reality game that accesses a smartphone's GPS and camera to allow players to explore their surroundings in search of more than 100 Pokemon creatures.
Features within the game include "Pokestops" — where users collect items they can use — and gyms, which are larger landmarks that users battle to control.
The game has been credited with getting people off of their couches, but also has resulted in many unintended consequences, like muggings and car crashes.