Background: Children's friendship quality is a particularly important risk or protective factor for internalizing problems and loneliness. Past research indicates that relationship satisfaction is related to perceived similarity; however, it is unclear whether this relation is seen in girls' friendship quality and whether this relation is significant over and above internalizing problems and loneliness. Objective: To improve our understanding of the unique relation between girls' perceived similarity to unfamiliar and familiar peers and friendship quality. Methods: 116 girls aged 9-11 completed self-report questionnaires; we examined unique relations of girls' perceived similarity to expected friendship quality with three hypothetical unfamiliar girls (prosocial, withdrawn/depressed, and aggressive) and perceived friendship quality with a nominated friend. Results: Girls' perceived similarity to all three unfamiliar girls was related to higher expected positive friendship quality, but it was only related to expected negative friendship quality for the unfamiliar aggressive girl. Girls' perceived similarity to her nominated friend was uniquely related to perceived positive but not negative friendship quality. Dyadic loneliness was inversely related to girls' perceived positive friendship quality with her nominated friend. Depression and dyadic loneliness were both positively related to girls' perceived negative friendship quality with her nominated friend. Conclusions: Girls' perceived similarity to familiar and unfamiliar peers is uniquely related to friendship quality. Training, prevention, and intervention programs targeting internalizing problems, loneliness, and social problems could benefit from incorporating this information.