The newspaper Tokyo Shinbun said that the unnamed woman, in her 30s, was tricked into believing he was a single Japanese man who had recently graduated from Kyoto University.
After the child’s birth, however, she discovered that the father was a married Chinese man, according to the news outlet.
The woman, who was already the mother of one child, had wanted a larger family, but chose not to conceive with her husband, after it emerged he had a hereditary disease.
The pair searched for a donor online and found a suitable match in July 2019.
As artificial insemination is limited to heterosexual married couples in Japan, the woman decided to sleep with this man on 10 occasions in the hope of becoming pregnant.
Following her discovery about his identity, the woman took her baby to an adoption agency in Tokyo and also launched legal proceedings against the sperm donor, seeking £2.1 million (330m yen) in damages for the “emotional distress” he had caused her.
At a press conference on Tuesday, her lawyer said she suffered sleep disorders as a result of what had happened.
The Independent could not reach the lawyer, the mother or the donor for comment as they were all anonymous in Japanese media reports.