The financial losses for the fiscal year ending Sept. 30 were nearly $11 billion more than the previous year.
The numbers cap a tumultuous financial year in which the post office was forced for the first time to default on more than $11 billion in payments to avert bankruptcy.
The mail agency for months has been urging Congress to pass an overhaul bill to let it trim letter delivery to five days a week and reduce annual payments for future retiree health care.
Postmaster General Patrick Donahoe says the large losses cannot be sustained.
The mail agency forecasts billions in additional losses next year as it awaits financial relief from Congress.