The walkout affects more than 500 hospitals and medical facilities, mainly in California and Hawaii, including several in the Los Angeles area, according to local media reports, Xinhua reported.
The five-day strike began at 7 a.m. Pacific Time on Tuesday and is scheduled to end at 7 a.m. Sunday, according to a post on Kaiser Permanente's official website.
The action marks the largest strike in the 50-year history of the United Nurses Associations of California/Union of Health Care Professionals, the union representing the workers.
"Kaiser employees are launching a 5-day strike against Kaiser Permanente -- not because they want to, but because they have to," the union said in a media release. The strike aims to win safe staffing, fair pay, and protect patient care, it added.
In a statement, Kaiser Permanente called the strike "unnecessary and disruptive," saying the company remains committed to reaching an agreement that balances fair pay with high-quality, affordable care.