WASHINGTON—In an unusual act of independence, the U.S. Postal Service said Wednesday it will end Saturday mail delivery to homes and businesses beginning Aug. 5 to curb losses that ballooned to $15.9 billion in its most recent fiscal year.

The move sets up a potential confrontation with Congress, which in the past has forbidden the agency from ending Saturday delivery. It also could force businesses including newspapers and magazines to rethink how they reach customers on weekends.

...