Jonjon Crummie, 18, allegedly shot and killed his girlfriend inside his residence on the 1600 block of Raymond Avenue around 8:45 a.m.
Officers responded to reports of shots fired and found 19-year-old Irais Acosta dead at the scene from a single gunshot wound.
Crummie was last seen running northbound from the location and had been on the run since the shooting. He was arrested for homicide around 4 p.m. Monday.
Police say the suspect was transported to Los Angeles County Medical Center by his mother, who facilitated his surrender.
He was detained by the Los Angeles County Sheriff's Department pending the arrival of Pasadena detectives.
Police withheld Crummie's relationship to the victim, but family and friends of the victim identified Crummie as her boyfriend.
Acosta's friends remain in disbelief over her death.
"She didn't deserve it. She was crazy, spontaneous, she was outgoing. Her personality was too strong, like everybody in Pasadena knows her, everyone. She always made friends, she never made no enemies, no nothing," said Acosta's friend Rosa Cardenas.
According to her family, Acosta was planning on attending Pasadena City College to study nursing.
Acosta's father told Eyewitness News through an interpreter that his daughter was a beautiful person.
A candlelight vigil was held for Acosta at 830 North Garfield Avenue in Pasadena Monday around 5 p.m.
The victim's family has set up a fund to raise money for her funeral services. For more information, visit www.gofundme.com
Anyone with information regarding the shooting was urged to contact the Pasadena Police Department at (626) 744-4241.