The governor, a former emergency room doctor, was traveling by car through downtown Portland about 5 p.m. when he saw "someone along the edge of the street who seemed to be attempting to resuscitate a woman" lying on the ground, Kitzhaber spokeswoman Nkenge Harmon Johnson said in an email to The Oregonian.
The governor told his driver to pull over, asked his security officers to call 911, quickly assessed the woman and began CPR, Portland Fire and Rescue Lt. Rich Chatman said.
KGW-TV first reported the governor's rescue effort.
The woman had regained a pulse but was not breathing when a fire truck arrived within about four minutes of the 911 call. At that point, the governor had provided her with an oxygen mask for rescue breathing, said Lt. Alan Ferschweiler, a fire department medic at the scene.
Medics took over her treatment. She had started to breathe "and make purposeful movements" by the time an ambulance arrived to take her to a hospital, Ferschweiler said. Her condition was not known late Monday night.
The medic described the woman as being in her 30s.
When the 911 call came in, medics were told "the governor's there and he's doing CPR."
"We don't always believe everything we're told until we see it," Ferschweiler said in a telephone interview. "It was great to see him do his work and help out when his services were needed."
Kitzhaber has rescued people before.
In 2010, while he was at a gubernatorial debate in Eugene, the forum was suspended for about 20 minutes after an audience member suffered an apparent seizure.
When someone asked if there was a doctor in the house, Kitzhaber rushed up the aisle to provide first aid.
He checked the man's vital signs and ensured he had an open airway until paramedics arrived.
The 67-year-old Kitzhaber, a Democrat, was elected governor twice in the 1990s. He was barred by term limits from running in 2002, but made a political comeback and won an unprecedented third term in November 2010. He's running for a fourth term.