The 38-year-old singer, who is known for her girl-next-door image, was arrested on suspicion of possessing illegal stimulants.
Tokyo police said a small quantity of drugs had been found in her apartment.
Her 10-year-old son, who had also been missing, was found safe in Tokyo on Thursday night.
"Noriko Sakai was arrested last night and we would like to offer an apology to all her fans for the trouble caused," said Masahisa Aizawa, president of Sakai's management company, Sun Music.
"I would like her to reflect on what she has done and feel the weight of the crime she has committed.
"I would strongly urge her to seek rehabilitation," he added.
Ms Sakai's husband, Yuichi Takaso, 41, was arrested last Monday for alleged drug possession.
Questions concerning the star's whereabouts have dominated headlines in Asia ever since, with her mother-in-law asking police to search for her, and Mr Aizawa urging his client not to go through this difficult time by herself.
Ms Sakai rose to fame in 1987 after releasing a single, Otoko no Ko ni Naritai, a few days before her 16th birthday.
Known as Nori-P during her early career, she began to use her real name after turning to acting in the 1990s.
One of her most acclaimed roles came in TV melodrama Hitotsu Yane no Shita.
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