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Objective Trisomy 18 is one of the major numerical chromosomal disorders. The incidence of trisomy 18 is approximately one in 6000 live births. Dandy–Walker malformation (DWM) is the most common congenital malformation of the cerebellum, with an incidence of about one in 5000 live births. The incidence of trisomy 18 associated with DWM is rare and long-term survival rate is very low. Case report A case involving a 39-year-old pregnant female with a case of trisomy 18 associated with DWM. Conclusion The incidence of trisomy 18 associated with DWM is rare, and our report presents an unusual case that supplements our knowledge of this condition. We report a case involving a 39-year-old pregnant female with a case of trisomy 18 associated with Dandy–Walker malformation (DWM). Fetal ultrasonography showed hypoplasia of the cerebellar vermis and dilatation of the fourth ventricle and was characterized by an enlarged posterior fossa. Fetal magnetic resonance imaging showed inferior vermian hypoplasia and a large posterior fossa cyst communicating with the fourth ventricle causing high insertion of the torcular herophili, which was compatible with DWM. Furthermore, the karyotyping report revealed trisomy 18. The incidence of trisomy 18 associated with DWM is rare, and our report presents an unusual case that supplements our knowledge of this condition.