Senator Cox, who was first elected to the upper house in 2021 to fill a vacancy for the Greens, said her values had aligned more with Labor.
She approached Prime Minister Anthony Albanese on an offer to join Labor and bolster its ranks in the Senate.
"I've worked hard to make Australia fairer and much more reconciled, but recently, I've lost some confidence in the capacity for the Greens to assist me in being able to progress this," Senator Cox told reporters in Perth on Monday.
"I feel reassured with my experience, my skills and my knowledge I can continue to collaborate and build on the existing relationships with an already amazing team of Labor MPs across the country."
Senator Cox had previously been a member of the Labor Party before joining the Greens, and will be formally admitted into Labor on Tuesday.
Mr Albanese said Senator Cox, the first Indigenous woman to represent the state in the upper house, would be a welcome addition to the government benches.
"She wants to be part of a team that's delivering progress for this country by being a part of a government that can make decisions, to make a difference," he said.
"Dorinda Cox is someone who's a former police officer. She's someone who has a lot to offer in terms of policy development."
The senator had been accused of bullying staffers in 2024, with as many as 20 employees leaving the office in a three-year period and complaints made with parliament's workplace support service.
The prime minister said the allegations had been examined.
"They were all dealt with in Senator Cox's case and dealt with appropriately," he said.
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