Supportive peer relationships are fundamental for mental health and well-being. Hence, peers and friends are a valuable resource, especially at the time of transition from primary to secondary school. Yet, current literature lacks both novel approaches to studying friendship development and how to involve early adolescents in research that is being conducted about them. Within the present study we used novel participatory research methods involving early adolescents who were active in the analysis of their own generated data. We aimed to better understand their perspectives on factors that facilitate and hinder friendship development with peers during the time of school transition between primary and secondary schools. A total of 916 pupils (Mage = 10.44 years, range = 9-16) participated in 54 participatory workshops that were conducted in Austria. We used reflexive thematic analysis to analyze qualitative data from portions of a large series of participatory workshop activities. Moreover, we actively involved participants in the analysis of their own generated data. Themes were structured into personal, interpersonal, and external factors. We found that early adolescents valued kind peers that (a) give them a feeling of safety, (b) show supportive and empathic actions, (c) manage conflicts, (d) avoid negative behavior, (e) spend time with them, and (f) communicate in the offline and online environments. Although shared norms of behavior can support friendship development, friendship jealousy and tolerating bigger friendship groups were identified as important potential barriers. Additionally, external factors (i.e., given circumstances), such as similarities, physical proximity, and duration of acquaintance were included in our data but were perceived as less important by early adolescents. Our results supplement the existing peer relationship literature by showing which factors early adolescents themselves chose as most relevant for friendship development. We conclude with a discussion regarding the implications for school psychology practice and future research.