Tram passengers reported that the sign appeared Monday morning and made the feelings of one homeowner crystal clear.
What isn't known is who the complainant is. No one answered when KATU reporters knocked on the door of the home.
The large, professionally-produced sign simply says "(expletive) THE TRAM" in large black letters against a white background.
The tram was the target of fierce opposition from homeowners who ended up living below the aerial transit. They cited privacy concerns as their main point of opposition.
As time wore on and the tram's construction costs began to balloon from $15 million to $57 million, some even called for a halt to construction.
The tram was eventually completed and began carrying passengers in January.
The lower section of the windows in the tram cars are frosted so passengers cannot look directly down into homes below, but the sign is still clearly visible.
The sign is not visible from the street, it is only visible from the air. Parents riding with children on the tram expressed concern over the expletive used on the sign.
The city attorney's office is looking into whether the sign violates any city ordinances, but a cursory look at city regulations revealed that the sign may be protected speech on private property.
City officials are continuing to investigate if the sign violates any statutes.