Friendships are beneficial to a person's growth and well-being. People in close relationships may experience four types of self-change: self-expansion, self-pruning, self-contraction, and self-adulteration. The current cross-sectional research sought to investigate whether these relational self-change processes explain the links between friendship quality and well-being in same-sex best friendships. Study 1A (N = 187) adapted the Turkish Relational Self-Change Scale to the friendship context and confirmed its psychometric adequacy in addressing friendship-based self-changes in Türkiye. Study 1B (N = 306) examined the links between different friendship functions and relational self-changes, as well as whether relational self-changes explain the link between friendship quality and ontological well-being. Among the specific friendship functions, stimulating companionship and self-validation were related to self-expansion and self-pruning, whereas reliable alliance was related to self-contraction and self-adulteration. Self-adulteration also indirectly explained the relationship between friendship quality and ontological well-being. Study 2 (N = 204) revealed that only self-pruning explained the link between perceived friend responsiveness and overall well-being. The contributions of friendships to self-change and personal well-being were discussed in light of the findings.