Holtzclaw faced 36 counts. He was found guilty on 18.
The former officer cried openly in the courtroom and rocked in his chair as the verdict was being read. Jurors deliberated for more than 40 hours over four days.
The Oklahoma City Police Department welcomed the verdict. "We are satisfied with the jury's decision and firmly believe justice was served," it said.
Sentencing is set for next month.
His trial touched upon the explosive intersection of race, policing and justice in America.
Holtzclaw, whose father is white and mother Japanese, was accused of assaulting or raping 13 women, all black, while he was on the job. Court records identify his race as "Asian or Pacific Islander."
The jury was all-white, composed of eight men and four women.
These racial dimensions energized civil rights and women's activists to draw attention to the case. Before the verdict was read, Oklahoma City NAACP President Garland Pruitt was concerned about the jury in the case, he told CNN affiliate KOCO.
"We're very disappointed, very, very disappointed, that we don't have any minorities on there," Pruitt said. "We're not saying justice can't prevail, but we can be suspicious of it being (run) in a manner."
Benjamin Crump, president of the National Bar Association, was also monitoring the case. He represented the families of Trayvon Martin and Michael Brown.
Martin, a black teenager, was fatally shot by George Zimmerman, who was acquitted of charges in 2013. Brown was killed by an officer in Ferguson, Missouri, in 2014.
"We will be here to make sure that this is not swept under the rug," Crump told reporters after attending a portion of the trial last month. "We come here to stand with these 13 victims of rape, who happen to be African-American women, to say that their lives matter, too."
In the first pool of 24 potential jurors, there were three black men, but they were not picked for the jury. Two alternates have Spanish surnames. About 70 potential jurors were initially called in for the case, the affiliate reported. Prosecutors would not comment on the jury composition.
At the center of the case was how all victims, ages 17 to 50s, had criminal histories of drug use or prostitution, according to The Oklahoman newspaper.
Prosecutors said Holtzclaw preyed upon the women in one of the state's poorest neighborhoods and used his badge and uniform during traffic stops to force the victims to submit to an escalating level of crimes, from groping to rape.
Holtzclaw was accused of manipulating the victims by promising to drop a drug charge against them if they didn't report the purported assaults.
"I didn't think anyone would believe me. I'm a black female," one accuser testified in the trial, according to the newspaper. She alleged Holtzclaw touched her bare breasts and later went to her home to force her to expose her genitals and breasts.
But defense attorneys Scott Adams and Robert Gray painted a different picture of the accusers, saying the women were "street-smart like you can't imagine" according to CNN affiliate KFOR.
The defense argued some of the alleged victims were high when the purported assaults took place.
During the trial, one accuser was removed from the stand until she sobered up after she told the court, "I'm not going to lie. Before I came here, I smoked some marijuana and a blunt stick laced with PCP," The Oklahoman reported.
She alleged that Holtzclaw touched her bare breasts and forced her to expose her genitals in 2014.
Holtzclaw, who turned 29 on Thursday, had his own campaign for justice, with a Facebook page
posted by his family.
Holtzclaw was a star middle linebacker on the Eastern Michigan University football team
and graduated with a degree in criminal justice. His father is a lieutenant on the Enid Police Department, the family said. He is also the brother-in-law of a law enforcement officer, The Oklahoman reported.
In all, Holtzclaw faced 36 charges, including burglary, stalking, indecent exposure, sexual battery, forcible oral sodomy and rape.
The case began after one woman came forward and accused Holtzclaw of "sexual impropriety" during a traffic stop, Oklahoma City police Capt. Dexter Nelson told CNN last year.
It snowballed from there.
Holtzclaw was fired from the force after allegations surfaced in 2014, which led to an internal investigation.
In a termination letter obtained by KFOR, his former boss, Chief William Citty, wrote of the alleged offenses against women, "The greatest abuse of police authority I have witnessed in my 37 years as a member of this agency."