The report describes a unique case of newborn conjoined twins in guinea pigs (Cavia aperea f. porcellus). Connected twins in guinea pigs have been very rare, and there are only a few previous reported cases. The newborns were stillborn in breeding in the Czech Republic in 2020 as two bodies conjoined into one. The external and internal structure of the body and the type of connection of both of the bodies are described. The weights of selected internal organs of the twins are compared with those of normally developed newborn guinea pigs.