<p>Plastic debris entanglement is a significant threat to avian populations. While primarily reported in marine species, terrestrial birds inhabiting urban environments are increasingly exposed to anthropogenic materials. We studied mortality and injury in the Eurasian tree sparrow (<i>Passer montanus</i>) resulting from the incorporation of plastic debris into nests across an urbanization gradient near Gwangju, Republic of Korea. We quantified urbanization based on building, paved road, and vegetation cover using semi-automated image processing. All 38 nests surveyed across 11 roof-tiled houses contained anthropogenic materials, constituting an average of 13.6% (6.51 ± 5.31&#xa0;g) of the total dry nest weight. Linear materials, particularly plastic line, were the most prevalent type, used mainly in the outer wall of the nest. We documented a total of seven carcasses (both juvenile and adult) and three live nestlings entangled with this material. Entanglement primarily occurred around the legs, wings, and/or neck, leading to death for the deceased individuals and injury for the surviving nestlings. The higher the urbanization score, the greater the dry weight of anthropogenic material within the nest. Although the live nestlings were rescued and successfully fledged, the high incidence of mortality indicates that a severe ecological trap is created by the widespread availability and use of linear plastic debris as nesting material. These findings highlight a need for increased conservation focus on the entanglement risk posed by plastic debris to common, urban-adapted terrestrial bird species.</p>