The Shibuya ward in Tokyo has become the first place in Japan to recognise same-sex partnerships.
It passed an ordinance allowing officials to issue certificates to same-sex couples this summer.
The documents are not legally binding but the ward is calling on businesses to recognise the certificates and accord equal treatment.
Japan remains a conservative country but is seeing social shifts on issues such as gay rights.
The nation's constitution identifies marriage as a union between "both sexes".
Though the country remains tolerant of homosexuality, same-sex couples do not get legal protection.
Normal visiting rights do not apply in hospitals, and they may be refused a tenancy from landlords because their relationship is not recognised.
Shibuya, which has 217,000 residents, is a trendy area known for its creativity and liberalism. Its mayor Toshitake Kuwahara told reporters previously that the move was in keeping with the area's character .
"The purpose is to realise a society where everyone can live in hope," he said.
Gay rights activists celebrated the move on Tuesday, with a small group holding up a banner thanking officials.
"To marry the same sex is no different from marrying the opposite sex,'' said Koyuki Higashi, who appeared with her partner Hiroko Masuhara.
The BBC's Rupert Wingfield-Hayes in Tokyo says that the ordinance amounts to a moral obligation on Shibuya businesses, which will not be penalised if they do not recognise the certificate.
Violators, however, will have their names posted on the ward's website. The district is also planning an educational campaign on gay rights issues.
The ordinance was supported by the majority of the area's legislators, but not by representatives of the governing right-wing Liberal Democratic Party (LDP).
The LDP is headed by Prime Minister Shinzo Abe, who happens to live in Shibuya. His party has come out strongly against the ordinance as undermining the foundations of society, says our correspondent.
A recent survey by the newspaper Sankei and television network Fuji News found that more than half of those questioned would support legalising same-sex partnerships and nearly 60% supported the ordinance in Shibuya.