Phytomyza impunita sp. n. is described in the subgenus Ptochomyza Hering. This species is known from a single location in the Republic of South Africa, representing the fifth described species in the subgenus, and the second known Ptochomyza to occur outside of the Palaearctic Region, with P. asparagivora (Spencer) known from southern Europe, Pakistan, Ethiopia and Kenya. The new species is described and figured, and discussed in relation to congeners.