This cross-sectional study characterized the prevalence of enteritis associated with Serpinema microcephalus infections in the intestinal tracts of red-eared slider turtles (Trachemys scripta elegans), river cooters (Pseudemys concinna), false map turtles (Graptemys pseudogeographica), and Chinese stripe-necked turtles (Ocadia sinensis) in Spain. All four species (three American, one Asiatic) were introduced into Spain through the pet trade. S. microcephalus is an Old World nematode that has been found to parasitize both invasive and native turtles. This parasite causes a significant inflammatory reaction in the intestines of exotic turtles that is not usually seen in native species. Parasites were detected in 27/70 turtles examined (38.6%; 95% confidence interval: 27.2–51%). Total worm counts ranged from 5 to 21. The nematodes were always found in the enteric mucosa of the duodenum and jejunum. The lesions in the turtles varied but included catarrhal to ulcerative enteritis and secondary hemorrhage surrounding the parasitized areas.