Friendship experiences have been shown to be important predictors of adolescents’ loneliness. The current study examined selection and socialization effects of loneliness within reciprocal best friendships, while controlling for friendship quality. Analyses were conducted on a sample of 884 adolescents (42.08% boys), making up 442 dyads, who were on average 13.51 years old (SD = 1.37). Adolescents completed the peer-related loneliness subscale of the Loneliness and Aloneness Scale for Children and Adolescents as well as the Friendship Qualities Scale. A longitudinal actor-partner interdependence model, which accounts for the interdependencies in the data of best friends, suggested the presence of a selection effect for loneliness but no socialization effect. This finding within best friendships contrasts with studies on friendship networks where both selection and socialization were established.