Artificial debris, along with sea plants, were found as gut contents of an immature female green sea turtle (Chelonia mydas) with a shell length of 470 mm, stranded on the Kozu coast, Odawara, Kanagawa Prefecture. This artificial debris was examined, and items measured, to understand trends in the color and size of debris that green sea turtles mistakenly ingest. This study found that most of the artificial debris comprised soft plastic. These items were mostly transparent or semi-transparent, with the proportion of colored soft plastics being low. The other items of artificial debris were fibers (including fishing line), but there was no hard plastic. The maximum length of the soft plastic pieces was approximately 210 mm, with most fragments being smaller. A band of white plastic found in the coprodeum retained its original shape, suggesting that green sea turtles can ingest and later pass soft plastics of around 200 mm in length. It is not clear whether ingestion of artificial debris was the cause of the turtle’s death on the basis of poor nutritional status, as its relatively well-developed fat tissue suggests it was not in a particularly poor nutritional state. However, 19 holes were found on the bone of the skull that appeared to be barnacle traces, and one of these had penetrated the bone. These holes were probably made by barnacles that embedded themselves in the turtle, such as Platylepas hexastylos, a species commonly found on green sea turtles. Attachment of large numbers of Platylepas hexastylos to the skin of a sea turtle occurs primarily in sick animals and is therefore a general indicator of reduced health. In the green sea turtle under study, the relationship between plastic ingestion and deteriorating health is not clear, but the turtle may have died because of deteriorating health, allowing the barnacles to erode and penetrate the skull. Thus, it is important to monitor the gut contents of sea turtles stranding in Kanagawa Prefecture and report evidence of penetration by barnacles.