The robbery happened when the victims, a 19-year-old man and a 20-year-old woman, were exiting a car in the 2700 block of Range Avenue at around 5:30 p.m.
Two young men approached them, and one of the two pointed a gun at the victims and ordered them to give him money. He was also shouting the name of a gang, police said.
The two victims turned over cash and other items to the two thieves, who fled on foot.
Police quickly established an identity for one of the suspects, and obtained a search warrant for a residence in the 2500 block of Terry Road.
Tommy Chavez, 18, of Santa Rosa was arrested and booked into Sonoma County Jail for two counts of robbery and gang enhancements. His bail was set at $180,000.
On Saturday, police continuing the investigation and believed they had identified the second suspect.
A second search warrant was executed in the 3000 block of Coffey Lane, and police arrested a 17-year-old Santa Rosa resident.
He was booked into the Sonoma County Juvenile Justice Center for two counts of robbery and gang enhancement, police said.
Both suspects were found with replica guns and some of the victims' items, police said.