Nematodes, belonging to the family Thelastomatidae, are parasites of saprophytic terrestrial arthropods, mainly cockroaches. American cockroach Periplaneta americana (Linnaeus, 1758) has high environmental adaptability, and has been spread worldwide by human activity. There are several reports of thelastomatid parasitic nematodes of P americana in the world. Here, the thelastomatid nematode Hammerschmidtiella diesingi (Hammerschmidt, 1838) (Oxyuridomorpha: Thelastomatidae), isolated from P americana is recorded for the first time in Japan. Through morphometrics, DIC and SEM observations, we confirmed that specimens of the present study agree with previous records of H. diesingi. DNA sequencing of the partial D2/D3 LSU expansion segment of the 28S ribosomal RNA gene revealed that H. diesingi from Japan matches with H. diesingi from other countries. Our contributions are very useful and fundamental for further analysis of the cockroach and parasite relations.