A fossil egg case of Argonauta hians Lightfoot, 1786 was collected from the middle Pliocene Sado-wara Formation of the Miyazaki Group in Shintomi Town, Miyazaki Prefecture, southwest Japan. This is the second report of argonautid fossils from Pliocene formations in Japan and marks the earliest occurrence of living species of Argonautidae in the North Pacific region. Fossil planktonic foraminifers and calcareous nannofossils were examined to establish the biostratigraphy of the fossil-bearing sequence. The horizon yielding the specimen of A. hians corresponds to the planktonic foraminiferal zone N.21 and calcareous nannofossil zone CN12a (ca. 3.25–3.12 Ma).