The Oscar-winning actress was spotted out driving in New York on Tuesday.
Zeta-Jones, 43, bravely revealed her diagnosis in 2011, telling PEOPLE at the time, "This is a disorder that affects millions of people and I am one of them."
She continued, "If my revelation of having Bipolar II has encouraged one person to seek help, then it is worth it. There is no need to suffer silently and there is no shame in seeking help."
She was equally forthright about her decision to return to treatment. "Catherine has proactively checked into a health-care facility," said her rep, Cece Yorke. "Previously Catherine has said that she is committed to periodic care in order to manage her health in an optimum manner."
Meanwhile, her husband, Michael Douglas, who stars as the late, flamboaynt entertainer Liberace in the upcoming HBO film Behind the Candelabra, says he's proud of his wife.
"She's doing a really good job," he said, while in Cannes to promote the film, which premieres on HBO Sunday night. "I can't wait to see her. I'm proud of her."
Next up for Zeta-Jones is the movie Red 2, out July 19, costarring Bruce Willis and Anthony Hopkins.
On June 3, Douglas, who battled stage 4 throat cancer nearly three years ago, will be honored by the American Cancer Society with the first annual Marvin Hamlisch Memorial Award, honoring the prolific stage and screen composer who died last year at 68.