"They're taking a break," a source told the entertainment People magazine, which reported that neither star had filed for divorce or legal separation.
Zeta-Jones' publicist could not be reached for comment.
The A-list couple have not been photographed together since April, appearing on red carpets solo and vacationing separately.
The pair have faced a tumultuous few years, with the 68-year-old Douglas fighting off throat cancer and Zeta-Jones, 43, undergoing treatment for bipolar disorder.
Douglas made waves in June when he attributed his throat cancer to the sexually transmitted Human papillomavirus (HPV), saying he caught it through performing oral sex. He later backtracked, saying he did not know what caused his cancer.
The Welsh actress Zeta-Jones won an Academy Award in 2003 for her supporting role in "Chicago" and has appeared in films such as "Traffic" and "Ocean's Twelve."
Douglas is an actor and producer who has won two Oscars and four Golden Globes in a career spanning more than four decades.