Similar flags were hung from utility poles this week in Tigard, a few miles south of Portland, The Oregonian reported.
The latest flag was flying next to the Fremont Bridge over the Willamette River. Flag and balloon were attached to a rope anchored to a rock that police say must have been dumped into the river either from the bridge or a boat.
It took police about an hour to get the flag down. Sgt. Brian Schmautz, a spokesman for the Portland Police, said the flag and balloon were kept as evidence, but he said no crime may have been committed.
"This one has one foot in the free speech world and one foot in the stupid world," Schmautz said.
Tigard police have also had reports of fliers bearing swastikas left in mail boxes and dumped on the street.