Phyllonorycter adderis is a moth of the Gracillariidae family. It is found in south-western Rwanda. The habitat consists of montane wet, closed canopy forests at an altitude of about 1,800 metres. The length of the forewings is 2.85–2.96 mm. The forewing ground colour is ochreous with white markings. The hindwings are dark grey with a long fringe of the same shading as the hindwing. Adults are on wing in August.