© American Society of Parasitologists 2017. The filarial nematode Setaria bidentata was found in 10 of 31 fetuses of the red brocket deer (Mazama americana) from the Loreto region of the Peruvian Amazon. A total of 25 specimens were collected and morphologically identified as S. bidentata. Filarial nematodes were found in the peritoneal cavity of 9 deer fetuses and the thoracic cavity of 1 fetus. Most specimens were adult stage. In this report, we provide morphometric data for these filarial specimens. This is the first study to demonstrate prenatal S. bidentata infection in cervid fetuses. Also, the finding of S. bidentata in Peru expands the geographic range of this parasite.