A good Samaritan bravely stopped a man from raping a young woman in front of an 11-year-old girl at an East Harlem housing project, police sources said yesterday.

The suspect cornered the terrified pair in an elevator at the Woodrow Wilson houses at around 5:15 p.m. Saturday and exposed himself, cops said.

William Green, 29, who lives at the East 105th Street building where the incident took place, then allegedly threatened the 23-year-old woman with rape, according to the sources.

But a neighbor rushed to her aid and thwarted Green’s alleged advances.