Sources have said that Zeta-Jones' re-entry into treatment is part of her plan to keep her mental illness in check.
"It's maintenance," one source told the gossip site about the 43-year-old actress' plans to complete a 30-day program.
"There was no big problem," a source told People. "This was just a good time to do it. She is in between projects. This has always been part of the plan."
"Catherine has proactively checked into a health care facility," Zeta-Jones' rep said. "Previously Catherine has said that she is committed to periodic care in order to manage her health in an optimum manner."
Zeta-Jones previously spend time at a Connecticut treatment facility in 2011, telling People at the time that she hoped more people with the illness would be encouraged to seek help.
"There is no need to suffer silently and there is no shame in seeking help," she said.
The National Institute of Mental Health defines bipolar disorder as a "manic-depressive illness" that causes dramatic shifts in mood and periods of depression.