The lawsuit claims the service's policy of delivering mail in a single unsecured bag for an entire building causes residents to lose crucial government checks, medical information and personal letters when their mail is mislaid or stolen.
Single-room-occupancy buildings, known as SROs, are sometimes called residential hotels.
Tenants typically rent a single room and share bathrooms and kitchens with other tenants. The lawsuit says SROs are sometimes the only housing that low-income or disabled people can afford. An estimated 30,000 San Franciscans live in such units.
City Attorney Dennis Herrera, who filed the lawsuit in U.S. District Court in San Francisco, said San Francisco Postmaster Noemi Luna announced in December that the service had to end delivery to individual SRO mailboxes because of "current fiscal shortages."
Herrera said, "Forcing our poorest residents to bear the brunt of budget cutbacks is not only immoral -- in this case, it's also illegal." The lawsuit claims the policy discriminates against SRO residents by denying them the same service as apartment residents.
Postal Service spokesman James Wigdel said he could not comment specifically on the lawsuit.
But Wigdel said that in general, "The policy of the Postal Service for hotels and hotel-like dwellings is single drop delivery."
The dispute appears to center on whether the SRO buildings are defined as apartments or hotels. The lawsuit claims the SROs fit squarely into the definition of apartments in Postal Service regulations.
The city was joined in filing by the lawsuit by three housing rights groups: the Central City SRO Collaborative, San Francisco Tenants Union and Housings Rights Committee of San Francisco.
The lawsuit asks for an injunction requiring the service to deliver mail to individual, locked mailboxes for SRO residents.