Catherine Zeta-Jones -- Enters Treatment Center for Bipolar Disorder

Catherine Zeta-Jones has checked in to a treatment center to get more help for her Bipolar disorder ... TMZ has learned.

We've learned ... the 43-year-old Academy Award winner checked in to the treatment center Monday ... and is expected to complete a 30-day program.

Sources connected to the actress tell us ... Jones' stint in the treatment center is a proactive measure. As one source puts it, "It's maintenance."

FYI -- Jones completed a stint at a treatment center in 2011 -- at the time, her rep said Catherine had "made the decision to check in to a mental health facility for a brief stay to treat her Bipolar II disorder."

Zeta-Jones was last seen in public on April 22 -- when she hit the red carpet at the 40th Anniversary Chaplin Award Gala at Lincoln Center looking healthy ... with her husband Michael Douglas by her side.